Both wet and dry detention basins are commonly used for the treatment of road runoff. The basin investigated in this study was located on the eastern side of the southern expressway in Adelaide, Australia. Event mean pollutant concentrations (EMCs) and total pollutant loads were determined for the basin inflows and outflows. The water quality parameters tested included pH, conductivity, total suspended solids, total dissolved solids (TDS), total Kjeldahl nitrogen, total phosphorous, total hydrocarbons, cadmium, chromium, copper, lead, zinc and nickel. Six critical events were sampled in this manner during the course of the investigation. Total pollutant loads passing through as basin inflow were determined over the six monitored events. Comparison of basin inflow and outflow quality demonstrated that the average load reductions varied from as little as 18% for TDS up to 77% for total phosphorous and lead. While copper concentrations at the basin inflow were detected in four out of the six events, no detectable concentrations were found in the outflows. The average reductions for lead and zinc were more than 50% on more than 80% of occasions.