A mathematical model of an automobile's emission rate is described. This model can be used to calculate the amounts of hydrocarbons, carbon monoxide, and oxides of nitrogen emitted by individual or groups of automobiles being driven over any known driving sequence. The development of the model requires the amounts of three pollutants given off by individual automobiles over short duration driving sequences (modes).The validity of the model is investigated by using it to calculate the amounts of each pollutant given off by individual automobiles over the hot transient portion (first 505 s) of the Federal Test Procedure driving sequence. These predicted emissions are then compared with observed amounts emitted from each automobile. Further, the ability of the model to predict emissions is investigated in light of the reproducibility of actual automobile emissions measured in replicated tests. These analyses indicate that the model performs extremely well. The model is intended to be used to predict emissions from automobiles being operated within the ranges of speed and acceleration covered in the input emission data.