The purpose of this study is to evaluate the effect of gasoline compositions and properties on vehicle tailpipe emissions in order to propose the automotive gasoline specifications in Thailand to be revised by the year 2000. A total of 270 exhaust emissions measurements were conducted using fifteen fuels according to Thailand emission standard test, TISI 1280 - 2538 (91/441/EEC). Six cars with different fuel supply systems or emission control devices were selected for the test based on the market share in Thailand. The following eight fuel parameters were examined: Reid Vapor Pressure (RVP), the 50 th percent distillation temperature (T50) and content of aromatics, olefins, benzene, MTBE, ethanol and sulfur. Total hydrocarbon (THC), carbon monoxide (CO), oxides of nitrogen (NOx) and toxic air pollutants such as benzene, 1,3-butadiene and carbonyl compounds were analyzed.Adding oxygenates into fuel decreased THC, CO emissions but increased NO x, aldehyde emissions and fuel consumption (l/km). Reducing aromatic content resulted in decrease of THC, NOx and benzene emissions. Benzene emission was decreased as benzene in fuel decreased. For vehicles equipped with catalytic converters, reduction of fuel sulfur reduced THC, CO and aldehyde emissions. This confirmed that fuel sulfur affects catalyst efficiency.