An efficient method has been developed for the Barbier-Grignard-type alkylation reaction of aldehydes (including aliphatic version) using unactivated alkyl halides in water in the presence of an In/CuI/I(2) or In/AgI/I(2) system. The reactions proceeded more efficiently in water than in organic solvent. In, CuI or AgI, and I(2) were all essential for the efficient progress of the reactions. A radical-type reaction mechanism was studied and proposed by using 4-pentenal as substrate.