Abstract: A description is provided of a mixture which is preparable by allowing the reaction of a premixture of (av), methanesulfonic acid (bv), one or more esters of one or more phosphorous-oxygen acids and (cv) one or more silanes. The invention also concerns the use of said mixtures as additives for the polyisocyanate component of a two-component binding agent system for preparation of a polyurethane resin. The invention further concerns a solution containing polyisocyanate for use as a component of a molding material binding agent system, and the use of a solution containing polyisocyanate as the polyisocyanate component of a two-component binding agent system for preparation of a polyurethane resin and corresponding two-component binding agent systems for preparation of a polyurethane resin.