Tributyltin methoxide and bistributyltin oxide react at the carbonyl group of many aldehydes and of certain more reactive ketones to give the 1 : 1 adducts (A). In the case of chloral and of 1,1,3-trichloro-1,3,3-trifluoropropane-2-one, the n.m.r. spectra show that 1 : 2 adducts (B) can also be formed, and, with chloral, repetition of this process gives the polymer (C). R3Sn·OR[graphic omitted]R3Sn·O·CR″R″′·OR′[graphic omitted] (A) R3Sn·[O·CR″R″′]2·OR′[graphic omitted]R3Sn·[O·CR″R″′]a·OR′(B)(C) The 1 : 1 adduct (A) can alternatively react with other carbonyl compounds by a reversible exchange of the organotin reagent. Acetyl chloride or methanol similarly react by displacing the carbonyl compound OCR″R‴. If one or both of the groups R″ and R‴ is a trihalogenomethyl group, methanolysis can alternatively involve displacement of the carbonyl compound R″CO·OR′, liberating the haloform; in consequence, organotin compounds catalyse the methanolysis of hexachloroacetone to methyl trichloroacetate and chloroform. It is suggested that all these reactions proceed through a 6-membered cyclic transition state.