Modal analyses have been performed on engine-out and tailpipe hydrocarbon and carbon monoxide mass emissions to help understand why fuels with increasing amounts of heavy hydrocarbon constituents produce significantly higher tailpipe hydrocarbon emissions, yet do not produce significantly higher tailpipe carbon monoxide emissions. Mass emissions were acquired for a fleet of ten 1989 model year vehicles operating on twenty six fuels of differing heavy hydrocarbon composition. These fuels formed two statistically designed matrices: one examining the effects of medium, heavy, and tail reformate and medium and heavy catalytically cracked components; and the other examining the effects of heavy paraffinic versus heavy aromatic components and the effects of the 50% distillation temperature.The modal analyses showed that the significantly higher tailpipe hydrocarbon emissions from fuels with high concentrations of heavy hydrocarbons result primarily from these fuels producing significantly higher engine-out hydrocarbon emissions during the first cycle of the Federal Test Procedure (FTP). During the remainder of the FTP, these fuels produce a modest and consistent increase in engine-out emissions. Since the catalytic converters are only just becoming active during the first cycle, these higher engine-out emissions are passed on, significantly increasing the tailpipe emissions. The other cycles of the FTP contribute slightly through an increase in engine-out emissions and, more importantly, a decrease in conversion efficiency. Carbon monoxide tailpipe emissions do not increase significantly with heavy-hydrocarbon fuels because the engine-out emissions of the first cycle are not abnormally increased.