A microassay for the measurement of sulfate sensitive to below 1 nmol is described. Accuracy and reproducibility were demonstrated with trichloroacetic acid clarified plant sample volumes of only 5 μl. The sensitivity of this assay enabled, for the first time, the measurement of sulfate in the medium of plant suspension cultures during its depletion down to the compensation point in a non‐disruptive manner. This was possible because very small volumes could be sampled from continuously growing cultures.