Abstract Twelve road-deposited sediment samples were analyzed for platinum-group elements (PGEs) and Pb in the <63 μm fraction of an urban watershed in Hawaii. Three samples were further fractionated into five size classes, from 63–125 μm to 1000–2000 μm, and these were analyzed for PGEs and Pb. Concentrations in the <63 μm fraction reached 174 μg/kg (Pt), 101 μg/kg (Pd), 16 μg/kg (Rh), and 1.3 μg/kg (Ir). Enrichment ratios followed the sequence Rh > Pt = Pd > Ir. Iridium was geogenic in origin, while the remaining PGEs indicated significant anthropogenic contamination. Palladium, Pt and Rh concentrations and enrichment signals were consistent with PGE bivariate ratios and PGE partitioning in three-way catalysts. Size partitioning indicated that the <63 μm fraction had the lowest PGE concentrations and mass loading percentages. These data suggest that autocatalyst PGE flux estimates into the environment will be significantly underestimated if only a fine grain size fraction is analyzed. Grain size fractionation of road sediments indicates that fluxes of platinum group elements into the environment from autocatalysts may have been significantly underestimated.