The present invention relates to solutions of isocyanato-isocyanurates based on mixtures of (i) 2,4- and/or 2,6-diisocyanato-toluene and (ii) 2,2'-, 2,4'- and/or 4,4'-diisocyanato-diphenyl methane in a molar ratio of (i) to (ii) of about 5:1 to 1:3, in plasticizers for polyvinyl chloride wherein the isocyanato-isocyanurates comprise about 1 to 50% by weight of the solution. The present invention also relates to a process for preparing these solutions which comprise partially trimerizing the isocyanate groups of the above diisocyanates using catalysts which accelerate the trimerization of isocyanate groups, the reaction either being conducted in the presence of plasticizers for polyvinyl chloride or the reaction product being dissolved in a plasticizer of this type, optionally after the separation of excess, unreacted diisocyanate.In addition, the present invention relates to the use of these solutions as adhesion-improving additives for coating compositions based on polyvinyl chloride. Besides improved adhesion, these compositions also demonstrate improved resistance to discoloration during thermal processing.