Abstract: A process for controlling the rate of addition of silicon bonded hydrogen atoms to organopolysiloxanes containing aliphatic unsaturation which comprises mixing (1) an organopolysiloxane containing silicon bonded hydrogen atoms and/or (2) an organopolysiloxane containing at least 50 silicon atoms per molecule and having aliphatically unsaturated groups with at least one organosilicon compound of the formulaCH.sub.2 .dbd.CHR.sub.2 SiO(SiR.sub.2 O).sub.n SiR.sub.2 CH.dbd.CH.sub.2in which R is a hydrocarbon radical or a substituted hydrocarbon radical free of aliphatic unsaturation and n is a number of from 0 to 6 prior to combining the organopolysiloxanes (1) and (2) in the presence of a platinum catalyst. The composition thus obtained has a substantially longer pot-life while only slightly extending the curing time.