Are you trimming them or letting them flower? Usually yellow leaves are a sign of the end of the growing cycle. I cut my chives weekly and get new green growth constantly. Usually if you have a patch of chives that you don't divide every other year the leaves will yellow quicker because you have more old growth than new.

If your chives have been in the same pot for more than a couple of years, turn them out and divide them and repot separately in fresh potting compost- you can wait till autumn to do it if you want.

Thanks. Yes, they've been in the pot 5 years with a wee bit of fresh compost added on top each year. Also I don't use them that regularly as there are just so many! I give loads of the plants away but haven't actually repotted the whole tubful. Guess I'll have to get busy. :-)