The invention concerns a method for preparing catalytic assemblies by mixing a metal catalyst capable of catalyzing a hydrosylilation reaction and an organo-phosphorus inhibitor of formula (I) or of formula (VIII) P(OR)3, either by mixing the inhibitor in a catalyst solution in an unsaturated silane or siloxane, or by mixing the inhibitor in a gum or oil, at a temperature higher than the melting or softening point of the organo-phosphorus compound, then in mixing the catalyst. The invention also concerns catalytic assemblies, a method for preparing single-constituent silicone compositions and the resulting compositions.