The instant invention is directed to a process for dissociating cellular and non-cellular polyurethane resins into re-usable starting products for the isocyanate polyaddition process, wherein the polyurethane is reacted with associates of lactams and adduct-formers having at least two Zerewitinoff-active hydrogen atoms at from 150.degree. to 250.degree. C., optionally at elevated pressure, the improvement wherein from 35 to 1000 parts, by weight, based on 100 parts, by weight, of the polyurethane, of an associate mixture of:(a) from 4 to 50 parts, by weight, of a lactam or azalactam, based on 100 parts, by weight, of polyurethane; and a mixture of:(b) water and/or a compound having a molecular weight of from 62 to 200 having at least two Zerewitinoff-active hydrogen atoms; and(c) a polyhydroxyl compound having a molecular weight of from 300 to 6000; is used, and the proportion by weight of components (b) to (c) is from 1:10 to 10:1, and wherein said component (a) is used in a quantity such that said 35 to 1000 parts comprises from 4 to 50 parts of component (a).