Time-temperature histories of fuel system components were measured on 80 passenger cars in the summer of 1968 in Los Angeles for several 24 h use patterns simulating normal vehicle operation. These included both driving segments on several types of highways and engine-off soak periods. Ambient temperature, type of driving, and vehicle design features have the greatest influences on fuel system temperatures. These time-temperature histories are of interest for fuel system evaporative emissions study. Two applications of these data have been reported.