A highly sensitive HPLC method for the simultaneous determination of soluble silicate and phosphate in environmental waters was developed, using ion-pair liquid chromatography preceded by the formation of their yellow α-heteropolymolybdates. The moderate-pH mobile phase enabled to use a highly efficient reversed-phase silica column. The pre-column coloring reactions at moderate-pH were reproducible for both silicate and phosphate in all quantification ranges with R.S.D.s less than 2% and 5%, respectively. The linear calibration lines between concentrations (mg-SiO 2 /L and mg-PO 4 /L) and peak area intensities were obtained for silicate and phosphate both with acceptable determination coefficients (r 2 ) of 0.9999. The limits of determination for both analytes were 0.007mg-SiO 2 /L and 0.003mg-PO 4 /L, which were calculated theoretically using 10σ/slope. The four-digit dynamic ranges were obtained for 0.007–10mg-SiO 2 /L and 0.003–20mg-PO 4 /L. The developed method was applied for the analysis of tap water, river water, coastal seawater, well water, hot-spring water, commercial mineral water, and laboratory water. The results were very reasonable and acceptable from the environmental viewpoints, which were well correlated with those confirmed by the molybdenum-blue spectrophotometry.