As others have noted, there appear to be 3 different types of HP 364XL black ink cartridge: CB321EE (800 pages); CN684EE (550 pages); CB322EE (photo ink). Armed with this knowledge from other useful Amazon reviews, I checked carefully that the item advertised and paid for was CB321EE. However, it was a CN684EE that arrived. After contacting the vendor they promptly refunded the £8.18 difference between what I had paid and the market value of the product I had actually received (gleaned from adverts for the lower capacity cartridge on Amazon). Clearly this ridiculous labelling system by HP is open to confusion and, potentially, mis-selling. I believe that HP should name the cartridges better to highlight the differences and that Amazon should insist on unambiguous advertising of which model is being sold via their website. Although disappointed to receive a lower capacity product than the one I ordered, I am heartened by the prompt response of the vendor in refunding the difference. The moral of the story: until HP improve their labelling, check carefully the precise product you are about to order; on receipt check that it is actually the one you ordered; if it isn't, contact the vendor immediately with a clear statement of what you want to happen, ie exchange, return or refund of difference. Finally, as others will have discovered before me, some apparently sexy and excellent value printers turn out to be very expensive if the ink doesn't last 2 minutes and cartridges are dear. I fear my HP Photosmart Plus falls into this category. Next time I replace my printer I will also compare prices of replacement ink before committing.
Well, after about 3 hours of trying to research which cartridge I should buy, (having read the reviews), I think I have finally got it.
There are 2 x Black 364XL cartridges and there are some unique differences. Don't be folled by the colour of the box as these do vary, however if you follow the steps below you should find the correct "XL" cartridge.
Part Number CB321EE: This cartridge is normal black. It looks thicker than the lower capacity black cartridge and is also thicker than the XL photo black. It also has a solid black "Pentagon" printed above the number. For my own printer, which is a B109n model, this is the correct one and looking above the slot where the cartidge fits into my printer, there is also a solid Black Pentagon. Also for reference, my own printer does not take seperate black cartridges for both photo and normal printing, ie, there is only a single slot for black.
Part Number CB322EE: This cartridge is for Photo Printing. Is is a thinner cartridge than the one above and is also the same thickness as the lower capacity "normal" black cartridge. This is a concern and is very confusing un less you know the differences. The main identifying point is that whereas the normal XL has a Pentagon on the front, this photo XL cartridge has a solid "Bow Tie" symbol on the front. It also has a picture of a camera on the packaging. I can only assume that this will not fit into my own printer due to the lack of this symbol around my cartridge housing.
It has been brought to our attention that HP have actually changed the capacity of the cartridge, under the same product code, the old one was an 800 page yield and the newer one is a 550 page yield. What a disgrace to do this without telling people who order & pay for the 800 page one & only receive the 550. and be £11 out of pocket
A lot of previous reviewers have had problems with these cartridges - for example, slow delivery, suspect authenticity, suspected re-filled cartridges, out of date stock and so on. One common denominator seems to be that they have bought them from third party suppliers trading through Amazon market place. I bought mine directly from Amazon and I'm pleased I did. They were genuine HP. They were new stock with a long expiry date (December 2013). They came with lightening speed. And, at the time I bought them, they were the cheapest on line anywhere. For the record (and after a bit of research) the capacity of the different sizes of HP 364 cartridges are as follows: standard black 6ml (250 pages); standard colour 3ml (300 pages); XL black 13ml (550 pages); XL colours 6ml 750 pages). I have no reason to think this combo pack contains anything less than the standard size cartridges
I received a cartridge only to find it's not the one as advertised on the Amazon web site, this should be: HP Original 363XL Black Ink Cartridge (High Capacity) CB321EE (800 Pages) The one they send is the cheaper CN684EE (500 Pages) I have now had a full refund because of false misleading advertising, the dealers explanation was HP have stopped making the large CB321EE (800 Pages) type, well my answer to that is change your web page advertisement! I am very surprised Amazon has not stopped this misleading web page be now
I have a recently purchased HP B110a all-in-one wireless printer which came with 'setup' ink cartridges. These don't last long so I needed all 4 colours. This pack is well priced at just over ][(individual cartridges are selling for  at my local discount stationary shop) and are made for my printer as stated. They arrived speedily despite free p&p and are well within the 'use by' date of 2012.