Process for preparing organotin esters

A process for preparing organotin esters of a carboxylic acid which comprises adding a base to an agitated mixture of (i) an organotin chloride of formula R'.sub.2 SnCl.sub.2 wherein each of R' is an organic group, and (ii) a carboxyl compound, which is a maleate half ester of formula R.sup.2 OOC--CH.dbd.CH--COOH wherein R.sup.2 is an alkyl group of 3-20 carbon atoms or a cyclohexyl group, the addition being carried out in the presence of added water, and producing a second mixture which is reacted to form an organotin product comprising an organotin ester with at least one maleate group attached to a tin atom, said product containing tin and chlorine with a weight ratio of chlorine to tin of less than 1:8, the amount of organotin chloride, carboxyl compound and base being such that for each mole of organotin chloride, there are 1.95-2.4 moles carboxyl compound and 1.9-2.5 equivalents of base, the proportion of the number of equivalents of base to equivalents of carboxyl compound being 0.9-1.3:1 and the proportion of equivalents of base to moles of added water being 0.001-0.5:1.