Indiumtrichloride promoted catalytically the addition of alkynyltins or allylictins to aldehydes in two ways: transmetallation with the tin compounds and activation of aldehydes as a Lewis acid. The stcreochcmical outcome in the addition of cinnamyltributyltin to an aldehyde strongly indicates that control of the process can be achieved simply by the choice of solvent acctonitrile or dichloromethane