A comprehensive method for predicting the mass and composition of evaporative losses from fuel tanks and carburetors and a computer program embodying the method are presented. This method was developed as one phase of the work on automotive evaporative emissions in the Inter-Industry Emission Control program*. It is presented as a tool to assist in the design and evaluation of fuels and vehicle evaporation control systems; it is not intended as a substitute for compliance testing. The prediction method makes use of published equations for carburetor loss, tank loss, and loss composition. Calculated losses are compared with measured losses from several published studies. The accuracy and the limitations of the method are described, and some relationships between losses and fuel and fuel system parameters are shown.