Phosphorous monitoring is important for eutrophication control in aquatic ecosystems, but ultratrace level concentrations may not be detected by conventional analytical methods. A method for measuring ultratrace level phosphate by online solid-phase extraction combined with HPLC was developed. A short column (50 mm) packed with octadecylsilane (ODS) was used for extraction of phosphoantimonylmolybdenum blue and dodecyltrimethylammonium hydrophobic ion-pair complexes. The ion-pair complexes entrapped on the ODS column were eluted with CH 3 CN/H 2 O (35/65; flow rate, 1.0 ml min-1) and monitored by an ultraviolet/visible spectrophotometer (=872 nm). Phosphate concentration was determined from the peak area of the ion pair. The limit of detection for orthophosphate was 0.15 μg PO 4 l-1 and the dynamic range was 0.15-100 μg PO 4 l-1. Although our method was susceptible to silicate interference, it could be corrected by a proposed interference correction equation.