In the manufacture of gasoline the front-end volatility needs to be carefully controlled in order to avoid hot fuel handling problems in service. Modern front-end volatility controls are intended primarily to provide protection against fuel pump vapour lock, which is believed to be the main field problem. In recognition of this a mathematical model has been developed which is capable of predicting the effects of changes in fuel composition and ambient temperature on the tendency of a car to exhibit fuel pump vapour lock problems. The model requires the fuel to be described by means of a low-resolution g.l.c. analysis, and an effort has been made to minimize the number of car characteristics needed for the computer simulation of the standard hot fuel handling test. Validation of the model for three car types with substantially different inlet system configurations has been achieved. The possibility of extending the model to cover hot fuel handling problems originating in the carburetter is discussed in a separate paper.