Using ambient concentrations of molecular markers, chemical mass balancing calculations have been performed to estimate the contribution of source categories to ambient fine particle levels at four sites in Houston, TX. Eight source profiles obtained using analytical methods equivalent to the techniques used in analysis of the ambient sample were used for the calculations. The chemical mass balancing model accurately reconstructed the measured concentrations of 24 molecular markers and three fine particle chemical components to estimate the contribution of each source to ambient fine particle loads. The results show that at three sites in the Houston urban area, diesel exhausts contribute between 1.6 and 3.7 μg m -3 to ambient fine particle levels, while at an upwind background site, diesel exhausts represent 0.5 μg m -3 of ambient fine particulate matter. Other important sources include gasoline-powered vehicles (1.1-2.8 μg m -3 at three urban sites and 0.5 μg m -3 at the background site); paved road dusts (1.0-2.8 μg m -3 urban and 0.1 μg m -3 background); meat cooking operations (0.9-1.3 μg m -3 urban and 0.7 μg m -3 background) and wood combustion (0.2-0.3 μg m -3 urban and <0.1 μg m -3 background). At one site located near the highly industrialized Houston Ship Channel, fuel oil combustion contributed an estimated 1.5 μg m -3, while fuel oil combustion was not an important contribution at the other sites. Model runs using seasonally averaged data showed a high variation in source strength between seasons for some sources (i.e. paved road dusts much higher in the spring and summer than in the winter), while other sources showed little or no seasonal variation (i.e. vehicle exhausts and meat cooking operations).