Partial least squares modeling as a powerful multivariate statistical tool was applied tothe simultaneous spectrophotometric determination of silicate and phosphate in aqueoussolutions. The concentration range for silicate and phosphate were 0.02-0.6 and 0.4-3 μg ml-1,respectively. The experimental calibration set was composed with 30 sample solutions using amixture design for two component mixtures. The absorption spectra were recorded from 500 to900 nm. The optimum conditions were obtained by simplex optimization method. The values ofroot mean square error of prediction (RMSEP) for silicate and phosphate using partial leastsquares (PLS) were 0.0017 and 0.02 μg ml-1, respectively. The effects of various cations andanions on detection of silicate and phosphate were investigated. The method was successfullyused for determination of silicate and phosphate in boiler water at power plant, well water andsewage samples.