A coal-derived light distillate and a petroleum-derived residuum have been studied by high-resolution mass spectrometry using both low-pressure ammonia chemical ionization and low-voltage electron impact ionization. A mass calibration mixture for use with ammonia chemical ionization has been developed. Selective ionization of the basic nitrogen-containing compounds by ammonia chemical ionization and compound type characterization of the resulting quasimolecular species has been demonstrated. Several homologous series of N-containing compounds were identified in a basic extract by electron impact ionization and compared with quasi-molecular analogs identified by ammonia chemical ionization.