a room temperature - curable organopolysiloxane composition which bonds well to the substrate in contact with it during curing , without requiring a special adhesion promoter , comprises an organopolysiloxane having at least 2 hydroxy or hydrocarbonoxy groups bonded to silicon , optionally a silane of the formula . sub . a sir . sup . 3 . sub . 4 - a and from 0 . 01 to 20 weight percent parts silyl ester of phosphoric acid in which the -- oh groups of phosphoric acid are replaced by -- osir 4 3 groups or silyl ester of polyphosphoric acid in which the -- oh groups of polyphosphoric acid are replaced by -- osir 4 3 groups , where r 4 is monovalent organic group .