Solid state C-13 NMR was used to characterize the complex of diphenylmethane 4,4′-diisocyanate (MDI) with acetone oxime as a potential crosslinking agent (PCA) for the aliphatic polyester–urethane block copolymers. The formation of MDI from the PCA was followed by heating the sample at 150°C. The active hydrogen atoms on the hard segments of the block copolymers can be made to react with the diisocyanate groups of the regenerated MDI from the PCA at the elevated temperature for the crosslinking. The regeneration of[BOND]N[DOUBLE BOND]C[DOUBLE BOND]O groups is also noted by the appearance of the 2270 cm−1 band in the FT-IR spectrum of the heat treated PCA.