In many industrial processes, pieces of the same polymer material are brought into contact at a temperature above the glass transition. Interdiffusion takes place across the interface and leads to a strengthening of the junction. Often, a cross-linker agent is also added in order to improve the global mechanical properties of the material, as in the formation of latex films from dispersed solutions of polymer particles. We studied theoretically the competition between the interdiffusion and the cross-linking reaction, and found that the control parameter tuning; the balance between these two processes is alpha = Q tau (0)A(0)*N(3)b(3)/N-e, where Q tau (0) accounts for the reactivity of the cross-linker, A(0)* is the initial concentration of sites capable of cross-linking on the polymer chains, N is the polymerization index, N, the number of segments between entanglements, and b a distance comparable to the segment length. The case of practical interest is alpha much less than 1: the reaction locks the interfacial chains once a significant mixing has developed, resulting in films with goad mechanical properties.