Abstract: A system is described and illustrated for flow injection spectrophotometric determination of Si. Powdered silicate rock (25 mg) is decomposed with 0.5 mL of HCl and 0.25 mL of HF by repeated (x3) heating and cooling for 1 and 5 min, respectively. After cooling and addition of 5 mL of 4% H3BO3 solution, the mixture is diluted with water. A 65 µL portion is injected into the carrier stream (0.6 M HCl, 2.45 mL min-1), which is then mixed with 3% (NH4)6Mo7O24 solution (2.45 mL min-1). The mixed solution then passes to a 200-cm reaction coil maintained at 45°C, and the absorbance is measured at 415 nm in a flow-through cell. The method is suitable for determination of 200 to 380 µg g-1 of Si. Results for determination of Si in 22 standard rock samples agreed with certified values. The coefficient of variation (n = 5) was 0.4%.