A range of carbonyl compds. including aliph. and arom. aldehydes and ketones were converted to the corresponding thioacetals in high yields in the presence of a catalytic amt. of hafnium trifluoromethanesulfonate (0.1 mol%, room temp.). The mild conditions tolerated various sensitive functional and protecting groups and were racemization-free when applied to alpha -aminoaldehydes. Transacetalization and chemoselective thioacetalization of arom. aldehydes in the presence of aliph. aldehydes and ketones were also documented. [on SciFinder (R)]