Inorganic phosphate was determined by the reduction of a phosphomolybdate complex with ascorbic acid or stannous chloride. Fluoride interfered with both methods: (i) Absorbance of the complex reduced by ascorbic acid decreased slowly after incubation with fluoride concentrations higher than 5 μM (0.01 ppm). When a large number of samples had to be handled it was difficult to control the influence of fluoride because after a certain time has elapsed between reduction of the complex and reading of the sample there was a decrease in absorbance. (ii) The complex reduced by stannous chloride also showed interference at fluoride concentrations higher than 0.5 μM (0.01 ppm). In this case the interference of fluoride was found to be predictable for a known F–– content. The presence of proteins caused precipitation problems with the ascorbic acid method whereas the stannous chloride method was less sensitive when proteins were pr