Rare earth metal trifluoromethanesulfonates (lanthanide and scandium triflates) were found to be stable Lewis acids in water and can act as Lewis acid catalysts in several carbon-carbon forming reactions In all cases, the reactions proceeded smoothly in the presence of a catalytic amount of the triflate under mild conditions in aqueous solution. For example, in the presence of a catalytic amount of rare earth metal triflate, formaldehyde in water solution (commercial formaldehyde solution) was activated and the hydroxymethylation reaction of silyl enol ethers proceeded smoothly Rare earth metal triflates were also quite effective in the aldol reaction of silyl enol ethers with aldehydes in aqueous media, and water-soluble aldehydes such as acetaldehyde, acrolein, and chloroacetaldehyde could be employed for direct use. Moreover, the allylation reaction of a wide variety of carbonyl compounds with tetraallyltin was successfully carried out in aqueous media by using scandium triflate as a catalyst. Unprotected sugars reacted directly to give the adducts in high yields. In all these reactions, lanthanide triflates were quantitatively recovered after the reactions were completed and could be reused.