Organotin esters from acid esters of di or poly carboxylic acids are made by mixing base an organotin halide and the acid ester in the presence of water, and reacting to form the organotin ester, the amount of base and acid ester being to provide 0.9-1.3 equivalents and at least 0.9 equivalents respectively per g. atom of halogen in the organotin halide, and the conditions being such as not to cause substantial hydrolysis, and the progressive mixing of base with organotin halide or acid ester being not later than the mixing of organotin halide and acid ester.