A sequential injection analysis system for the simultaneous determination of phosphate and silicate in waste water is proposed. The method is based on the formation of yellow vanadomolybdophosphate and molybdosilicate, respectively, in addition to the use of large sample volumes. The mutual interference between both analytes was eliminated by selection of the appropriate acidity and by sample segmentation with oxalic acid. The calibration graph for phosphate and silicate is linear up to 12 mg l-1 P and 30 mg l-1 Si, respectively. The detection limits are 0.2 mg l-1 P and 0.9 mg l-1 Si. The method provides a throughput of 23 samples h-1 with a relative standard deviation < 1.4% for phosphate and < 4% for silicate. The method was found to be suitable for the determination of these species in waste water samples.