Carbonyl sulfide (OCS) is thought to be the major precursor to the background stratospheric aerosol sulfate layer during nonvolcanic time periods. Long-term perturbations to this layer from increased OCS emissions could significantly influence the Earth's radiation budget, climate, and ozone levels. The present study was carried out in an effort to determine mass emission rates of OCS from automobiles, a potentially important global source of this gas. Studies were carried out on a variety of gasoline vehicles including those without catalytic converters, vehicles with older oxidation catalysts, and vehicles employing newer three-way catalysts. Preliminary measurements were also carried out on four diesel fuel cars and one medium-duty diesel fuel truck. Measurements of OCS were acquired by tunable diode laser absorption spectroscopy, and in most cases, measurements of CO were also acquired. Gasoline vehicles, which included some of the lowest and some of the highest CO emitters on the road today, revealed very high correlation between OCS and CO mass emission rates. The OCS-CO linear regression resulted in a slope of (5.8¿1.6)¿10-6(gOCS/gCO) and a correlation coefficient of 0.92. Our preliminary diesel fuel measurements resulted in a corresponding slope 34.5 times larger. On the basis of these results were calculated a global OCS source strength for gasoline and diesel fuel vehicles of 0.0008 to 0.008 Tg yr-1. The upper limit is a factor of 100 to 600 times less important than the sum of all OCS sources. In contrast to the global scale, automotive emissions of OCS may be important on a local scale, particularly when attempting to measure background concentrations and associated small secular trends. Our OCS-CO ratios have been shown to be very useful in helping to delineate automotive sources from other sources. ¿ American Geophysical Union 1992