PURPOSE: The titled composition that is obtained by mixing, in a specific proportion, a specific addition-reactive organosiloxane composition, a specific agent for giving thermal conductivity and a specific tackifier, thus being suitable for use in assembling heat-emitting electronic parts to heat-dissipating print base plates.

CONSTITUTION: The objective composition is obtained by combining (A) 100pts. wt. of an addition-reactive organosiloxane composition which is composed of a vinyl group-bearing organopolysiloxane, an organohydrodiene polysiloxane and a platinum catalyst, with (B) 50W300pts.wt. of an agent for giving thermal conductivity such as alumina, quartz powder, magnesia, boron nitride or silicon carbide and (C) 0.5W10pts.wt. of a tackifier such as aminosilane, epoxysilane or alkyl titanate.