Two major effects are observed during annealing. First, some cyanate groups hydrolyze into carbamate structures on Al and Au. The reaction is driven by the metal surface and both hydroxyl groups and adsorbed water can be involved. On Al, where the carbamate production is most prominent. a part of the cyanate groups in the prepolymer is expected to react with substrate hydroxyl groups. The resulting carbamate coupling to the oxide provides a new chemical adhesion mechanism. As a second effect, an unexpected strong loss of monomer molecules from the prepolymer layers is observed on both substrates at elevated temperatures. It is concluded that the monomer molecules are pushed out of the interphase layer with the substrates due to negative adsorption. Hence, the less mobile, bulky oligomers remain on the substrate and thermal curing of the adhesive must result in a polycyanurate network which is much weaker than in the bulk polymer. This could reduce the mechanical strength of an adhesive joint made with pure polycyanurates and could afford additional chemical modification for practical applications.