A class of firefighting foams, known as aqueous film-forming foam (AFFF), has the ability to form a film by spreading on the free surface of some hydrocarbon fuels. To find out whether this film could reduce the evaporation losses of volatile hydrocarbons, the evaporation rates of fuels covered with a film of AFFF were measured by weighing the fuel at different time intervals. The results proved that the addition of aqueous surfactant solutions on the fuels is not always synonymous to the reduction of evaporation rates, but sometimes it might also cause an increase. However, it was found that polyethylene glycol could modify AFFF in a way that it can form a protective permanent film on the surface of a fuel such as kerosene, thereby leading to a reduction of evaporation losses.