A reaction product is useful as a crosslinking agent for polymers having functional groups reactive with the reaction product such as hydroxyl, carboxylic acid, amide, thiol, urea, carbamate and/or thiocarbamate groups, wherein the polymer includes an acrylic, polyester, polyether and/or polyurethane. The reaction product is derived from a) a polyisocyanate or polyisocyanurate, b) a hydroxyalkyl carbamate, c) an aldehyde and d) at least one monohydric alcohol.