Evaporative emissions from automotive fuel systems are recognized as one cause of photochemical smog. A key factor in reducing evaporative emissions is to control the amount of fuel vapor generated in the fuel tank. This research focused on the fuel temperature in the tank which has a large effect on fuel vapor generation. Two factors causing the fuel temperature to rise are the heat received from the exhaust system and the heat received from returned fuel. An experimental method was developed for separating and evaluating these two heat quantities. An investigation was then made of the heat balance of the fuel system for several typical fuel tank layouts. It was found that the heat balance differed considerably depending on the fuel tank location. Using a theoretical heat balance model, an analysis was then made of the contributions of different factors to elevated fuel temperatures. The results revealed a direction to follow in designing technical measures for minimizing fuel vapor generation.