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 vehicles or groups of vehicles which are driven over arbitrary driving sequences. The model requires as input the amounts of the three pollutants emitted by individual automobiles over short duration driving sequences (modes) and therefore, the model is intended to be used to predict emissions from vehicles being operated within the ranges of speed and acceleration covered in the input emission data. The validity of the model has been investigated by predicting sections of the Federal Test Procedure and comparing predicted and actual values.