Abstract The flow injection principle was used in the photometric determination of calcium and magnesium with Eriochrome Black T as reagent. The reagent spectrum overlapped the absorption spectra of the complexes (520 nm), and a novel correction for both the dilution and the consumption of the reagent by the sample analytes is described for a single channel manifold. Correction is made either graphically or computationally using an iterative procedure. Sequential determinations were accomplished by measurements at pH 10.1 and 10.5, which results in differences in the relative absorbances of the clacium and magnesium complexes. At a sampling rate of 100–120 samples per hour with 20-μl sample injections, precision was ±2% relative standard deviation. The technique was demonstrated by the determination of calcium and magnesium in deproteinized serum samples.