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Just to add more oil, one can't upgrade the 2.5 GHz model to 16 GB, 8 GB is the limit, but one can upgrade the 2.7 GHz model to 16 GB.
It's a bit strange, but maybe Apple thought, buyers, who go for the lowest iMac don't have an interest in adding 16 GB RAM.
http://store.apple.com/us/browse/home/shop_mac/family/imac/select

Then again, EveryMac says, that that 2.5 GHz model has four slots.
http://www.everymac.com/systems/app...inch-aluminum-mid-2011-thunderbolt-specs.html

Anyway, what would be the added value by not including two fewer RAM slots? It would mean more work actually.

Base model has 4 slots - I've just added 2x4gb to give me 12gb - which works perfectly.

I have the base 2.5ghz 20.5" model - upped HD to 2TB and ram to 12gb
 
Just to add more oil, one can't upgrade the 2.5 GHz model to 16 GB, 8 GB is the limit, but one can upgrade the 2.7 GHz model to 16 GB.

One can't upgrade the 2.5 GHz model to 16 GB, but that's only on the Apple website. You can always buy memory elsewhere and upgrade it to 16 GB yourself.

Just because you can't do it on the website doesn't mean it's not possible in reality. Apple only does that because it wants you to get the 2.7 GHz model.
 
You can add up to 32gbs of ram on the base model yourself, just because apple doesn't let you on their website doesn't mean you can't do it yourself......

I guess, I should have specified, that the Apple Online Store doesn't let you. I know, that more is possible, just that Apple didn't want ignorant users to do so and probably lure them to the 2.7 GHz model.
 
I have the 2011 21.5" base 2.5 GHz iMac and it DEFINITELY has 4 slots, because I added 4x 4GB memory sticks myself.

I think the Apple quote is suggesting upgrading with 2 additional sticks @ 4 GB each to the original 2x 2GB, so four sticks total.

Thanks, that's what I was looking for. I didn't think it was true but it wouldn't surprise me if Apple did something like that.
 
I also want to hear from an actual person and not just a link.

I just upgraded mine a few weeks ago. It does have 4 and upgrading to 16gb works wonderfully. I have 16gb in mine and it only cost $80 total for the upgrade. Bought the ram from amazon with prime overnight shipping.
 
Thanks, that's what I was looking for. I didn't think it was true but it wouldn't surprise me if Apple did something like that.
If Apple reduced the number of slots to 2, it would be clearly documented on their site and in the User Guide. They would not change the specs without documenting that fact. There is simply no motivation to post inaccurate specifications for any model.
 
Not sure if this is too late, but I have a base model 21" (2011) iMac at 2.5GHz.

I can confirm that it has 4 slots.
 
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