The reaction between C-13-labeled maleic anhydride and Pittsburgh No. 8 coal has been investigated by NMR. The occurrence of the following reactions has been ruled out : ene reaction, maleic anhydride polymerization, ester formation, radical addition, electron transfer, and physical entrapment. Michael addition to -SH has been demonstrated in model systems, but does not occur with -OH groups and cannot explain the observed 10-20% (wt) incorporation of maleic anhydride. All data are consistent with the occurrence of Diels-Alder reactions. There are not enough reactive dienes in any coal structure model to yield the large maleic anhydride incorporations observed. Either current coal structures are seriously incorrect or this coal is exhibiting strongly enhanced reactivity whose origin is not understood.