PURPOSE: To produce a carboxyalkyl substd. organopolysiloxane from a silalactone compsn.

CONSTITUTION: A reaction mixture consisting of a halide salt catalyst selected from quaternary ammonium halide, quaternary phosphonium halide and the like and an ester represented by formula I (wherein X is chlorine or bromine; R is a monovalent hydrocarbon; Q is a divalent hydrocarbon and two or more carbon atoms in Q are present to separate a silicon atom and a carbon atom of a carbonyl group; and R' is alkyl) is heated under an anhydrous condition to obtain a silalactone compsn. having formula II (n is larger than 0) and this compsn. and water are mixed to form an organopolysiloxane containing at least one siloxane unit represented by formula III. The amt. of water is set so as to be sufficient to hydrolyze substantially all of hydrolyzable bonds bonded to silicon atoms in the silalactone compsn.