Can you have uneven memory in the two memory slots?

"To clarify things here:

Yes, you can have different sized SIMMs in the slots as long as each meets the needed speed for the given model. You may also be required into installing the smaller SIMM in a given slot. While you will have more memory to use the addressing will be slower.

If at all possible use the same sized SIMMs by the same manufacture (as some manufactures chips may not play together well) so you can use inter-leaved mode.

When you have the same sized SIMM in each slot the faster inter-leaved mode is used."

Well I have a late 2011 MacBook Pro 15 and I bought it at that time, 2011. Well, I did the 2gb in one slot which is still the apple standard and just replace the second slot with "ramjet" brand 8gb making it 10gb. Well, I've had that upgraded since last October 2013 and on may 20th 2014 after 8 months my MacBook Pro malfunctioned. I had to do the flat rate fix of $310 thru apple genious to have the logic board and both slot rams replaced back to 4gb ram. I lost 3.5 days. They gave me back in a static free envelope the aftermarket ramjet 8gb. They mentioned long term use of unmatched ram can put tremendous strain on the bigger ram causing constant over heating build up on part of the logic board it will start failing. I do a lot of heavy processing software like Photoshop CS5 and Final Cut Pro x and running lots of apps together. The apple agent said it's was like having unequal tire size. So they recommend equal match ram sets. I'm going try to get some compensation from ramjet. They said it was "absolutely" fine with miss match. Nope. I've had others tell me now should be matched.

CAN you have uneven RAM chips? Absolutely! Will it run? It should for a while at least. The problem is that your system will become unstable and likely crash regularly. The best option is to get matched RAM.

I’ve used on a Z68X board Two 4gb dimm and One 8gb dimm at the same time for a very long time… and no problems whatsoever…

what you're talking about off course you can my laptop is 4Gb and 8Gb Ram I'm using it since 6months ago with windows 10 and no problem I have noticed at all from using different size memory

"Hi Ethan,

I hope this no longer an issue for you however, as I understand it having unequal Ram size is not a huge problem, but you must make sure the Manufacturers are compatible, and operate at the same speed, or you will end up with instabilities with the system.

Such as, ""Crashes, Freezes, and eventually over longer periods may even do damage to the MotherBoard, where over-heating can possibly occur. The reason this can happen is because one slot is working overtime and there is often a tendency for the machine to run much slower than it is actually capable of doing...

This is just my understanding having used Macs since 2002, and having replaced most of the Ram, Hard drives and Batteries myself.

I am by no means an expert in the matter, however I have had to learn the hard way on more than 1 occasion, which I can assure you tends to make you think twice about doing things without a little research into the possible outcomes 1st, that said, ""if it aint broke, don't bother to fix it... Hope this has helped in some small way, Best of Luck Pal."