Unburned hydrocarbons (HCs), carbon monoxide (CO), and oxides of nitrogen (NOx) are the compounds regulated as pollutants by an environmental standard in the Metropolitan Area of Mexico City (MAMC). The main fuel used in vehicular transportation is gasoline, and the use of liquefied petroleum gas (LPG) is now an alternative as low emission technology to decrease the environmental impact of transportation operations. The environmental impact of commercial gasoline consumption in the Valley of Mexico was estimated by on-road and FTP-75 testing of three formulations of gasoline (one leaded [octane 81] and two unleaded [one octane 87 and one octane 93]). A fleet of 30 vehicles was used: 10 were chosen that had pre-1990 technology, while 12 were 1991-1996 vehicles equipped with fuel injection, catalytic converters, and air/ fuel ratio control technology. The remaining eight vehicles were high-performance new model vehicles (1995-1996) equipped with the newest technology available for pollution control. Fifteen vehicles in the fleet were also tested for the effect of changing from leaded to unleaded gasoline. Three different LPG formulations were tested using three vehicles representative of the LPG-powered fleet in the MAMC. Two gasoline-to-LPG conversion certified commercial systems were evaluated following the BAR-90 and the HOT-505 procedures. Emissions corresponding to the high-octane (premium) gasoline showed a 15% higher contribution to HCs with a 6% lower reactivity than the 87 octane gasoline; the HCs in the exhaust for premium gasoline are mainly isoparaffins. When the vehicles were tested on the road at high speeds, an average 3% increase in mileage was obtained when vehicles were switched from leaded to unleaded gasoline, while a 5% increase in mileage was observed when vehicles were switched from 87 octane to premium gasoline. The tests of LPG formulations indicated that a change in composition from 60% vol of propane to 85.5% vol reduces levels of HCs and CO emissions; such is not the case for the NOx emissions. The higher the concentration of propane, the higher the levels of NOx that reached values above the maximum limits set by the environmental standard. A value of 70% vol of propane in the LPG mixture, with variations no greater than 4%, seems to be the best method for reducing pollutant emissions in Mexico City.