Disclosed herein is a process for bonding substrates comprising applying a hot-melt adhesive to the surface of at least one of the substrates and contacting the surface of said substrate with the surface of the other substrate, said adhesive containing an ester group-containing polyisocyanate which is a solid below 30.degree. C. and liquid above 100.degree. C., corresponding to formula (I)A(OCO-B-NCO).sub.n (I)in whichn stands for a whole number or a fraction (statistical average) of from 2 to 6,A stands for an n-valent organic group such as is obtained by removal of hydroxyl groups from an n-valent polyhydroxyl compound in the molecular weight (Mn) range of from 800 to 6000 containing ester and/or carbonate groups andB stands for an aliphatic, cycloaliphatic or aromatic hydrocarbon group having up to 18 carbon atoms.