INTRODUCTION Evaporative emissions of gasoline constitute a significant contribution to the total hydrocarbons emitted to the atmosphere from automobiles without controls. Mechanical systems for controlling the major part of these evaporative emissions have been developed and have been installed on new vehicles sold in California starting with the 1970 model year. In 1971 they are being used on vehicles sold nationally. As yet@ there appears to be no inexpensive method for installing such systems on vehicles already in the hands of the public. As a result@ the rate at which evaporative emissions from motor vehicles will be lowered is governed by the rate of introduction of new vehicles. One possibility that has been considered for reducing evaporative emissions from older cars is to reduce the volatility of gasolines. In order to adequately consider such a possibility@ a relationship between fuel volatility parameters and evaporative emissions must be established@ then the benefits from reducing volatility must be weighed in terms of the effect on total atmospheric hydrocarbon loading@ finally the costs of reducing volatility must be considered. Reference 3 is an example of a comprehensive evaluation for one urban area. This report considers only the development of the relationships between evaporative emissions and fuel volatility parameters from cars without evaporative loss control devices. A number of volatility parameters affect the quantity of fuel evaporating from the carburetor and fuel tank under given conditions. Thus a single volatility parameter such as Reid Vapor Pressure does notadequately correlate the evaporative emissions for a vehicle population. A better approach is to develop a mathematical expression to combine the effect of controlling volatility parameters. With this expression the evaporative emissions from an uncontrolled vehicle population could be calculated. This report develops such an expression.