Process for preparing catalyst assemblies by mixing a metal catalyst capable of catalyzing a hydrosilylation reaction and an organophosphorus inhibitor of the formula (I) or of formula (VIII) P(OR)3, either by mixing the inhibitor into a solution of catalyst in an unsaturated silane or siloxane or by mixing the inhibitor into a gum or oil at a temperature greater than the melting temperature or softening temperature of the organophosphorus compound, then mixing the catalyst. Catalyst assemblies, process for preparing one-component silicone compositions, and resultant compositions.