In this paper we present a new fundamentally-based gasoline volatility parameter which is capable of predicting the cold-weather driveability of both hydrocarbon and oxygenate-containing fuels. This Enthalpy Requirement parameter is a measure of the heat input required to the fuel and air to achieve a given air/fuel vapour mixture strength in the inlet manifold. Enthalpy Requirement is calculated using the GLC-determined composition of the fuel and incorporates the volatilities, enthalpies of vaporization and stoichiometries of each individual component. This new parameter has been developed and tested using cold-weather driveability data obtained using a range of fuels with different volatilities in over 200 mainly carburetted vehicles. Enthalpy Requirement is a far superior predictor of the driveability of fuels containing 3%v methanol plus 2%v TBA or 10%v MTBE than is the ASTM distillation parameter E100 and is equally good as E100 at predicting the driveability of hydrocarbon fuels. Further tests on a smaller number of vehicles indicate that Enthalpy Requirement can also predict the driveability of fuels containing up to 20%v methanol.