A novel and efficient method is disclosed for decreasing the content of residual alkoxy groups in an organopolysiloxane such as so-called MQ resins, usually and unavoidably, having a considerable amount of residual alkoxy groups bonded to the silicon atoms to be very detrimental against practical application of the organopolysiloxane. The method comprises admixing the alkoxy-containing organopolysiloxane with a strongly alkaline compound such as alkali metal hydroxides as a catalyst, an aprotic polar solvent such as N,N-dimethyl formamide, dimethyl sulfoxide and the like and water each in a specified amount and heating the mixture, for example, under reflux so that the alkoxy groups are rapidly hydrolyzed and can be removed in the form of an alcohol.