A first-derivative spectrophotometric method utilizing the zero-crossing measurement technique was developed for the simultaneous determination of phosphate and silicate in synthetic detergents and waters. Two procedures, based on the formation of phospho- and silicomolybdenum blue complexes in the presence of metol and ascorbic acid, were adopted to allow the simultaneous determination of both anions. The P concentration ranges and detection limits for the determination of phosphate in the presence of silicate utilizing the metol and ascorbic acid procedures were 0.12–1.7 and 0.07–1.2 μg mL⁻¹ and 0.07 and 0.02 μg mL⁻¹, respectively. The corresponding Si concentration ranges and detection limits for the determination of silicate in the presence of phosphate were 0.12–1.5 and 0.08–1.3 and μg mL⁻¹ and 0.06 and 0.02 μg mL⁻¹, respectively. The relative standard deviations (n = 10) were 0.86–2.8% for 0.43 μg mL⁻¹ P and 0.5 μg mL⁻¹ Si. The proposed procedures were evaluated by a statistical analysis of the analytical data.