Thiazolidinediones have been used as aldol donors with aromatic aldehydes “on water” and in DMSO without using any catalyst to give the corresponding β-hydroxy carbonyl compounds in high yield and purity. The p-cyano and p-nitro substituted aldol products undergo syn/anti isomerization via an enolization mechanism providing an “on water” diastereoselectivity switch. Water molecules play a significant role in stabilizing the syn aldol products of pyridine and thiazole containing aldehydes.