Heat curable organopolysiloxane compositions comprising an organopolysiloxane having Si-bonded aliphatically unsaturated groups, an organohydrogenpolysiloxane having Si-­bonded hydrogen atoms, a platinum catalyst capable of promoting the addition of Si-bonded hydrogen to Si-bonded aliphatically unsaturated groups and a hydrazone compound. The resultant composition is stable at room temperature, but when heated to an elevated temperature cures to an elastomer.