My 23 month old son gets pretty bad diaper rashes any time he drinks anything besides milk and water it seems like. I usually just buy the tropicana juice and that gave him diaper rashes, then I tried to dilute it a lot (approximately 80% water, 20% juice) and that didn't really help much. Then we went back to nothing but milk again because milk doesn't give him diaper rashes. Within the past few days we have tried giving him gatorade and he started getting rashes again today.

Any ideas on what is causing this?

Not sure, but milk and water seems fine to drink. Unless you squeeze juice from fruit itself, juice isn't really natural, so perhaps the chemicals in it may be reacting with your child. You could always try something organic too. Some people are sensitive to the chemicals and additives in our foods, this could be the case?

I think it has to do with the acid found in the juice. Has your son had pasta with sauce? Sauce also contains acid.