The Thermocurable compositions which contain(A) at least one mercaptoalkyl-containing compound which contains at least two SiC-bonded radicals per molecule of the general formulaHS--R.sup.2 -- (I) where R.sup.2 represents a divalent hydrocarbon radical having from 1 to 8 carbon atom(s) per radical, selected from the group consisting mercaptoalkyl-containing organopolysiloxanes and cocondensates of mercaptoalkyl-containing organosilicon compounds containing groups capable of condensation, and hydroxyl-containing organic compounds,(B) at least one vinyl-containing compound which is compatible with constituent (A) and contains at least two groups per molecule of the general formulaR.sup.4 CH.dbd.CR.sup.3 C(O)O-- (II) where R.sup.3 represents a hydrogen atom or a methyl radical, and R.sup.4 represents a hydrogen atom or an alkyl radical having from 1 to 3 carbon atom(s) per radical, and(C) a mixture of a strong organic base selected from the group consisting of quaternary ammonium hydroxides, quaternary ammonium alkoxides, tertiary, secondary and primary amines, and a weak protonic acid.