Abstract Eriochrome black T can be employed as an indicator for free magnesium ions in concentrations of from 6 × 10 −5 to 2 × 10 −2 M. Calcium does not interfere. The determination is strongly dependent on pH, ionic strength, and temperature. These variables are examined particularly as sources of error. It is concluded that measurements of ±5–10% precision are readily attainable. The assay can be performed in the presence of protein in concentrations <0.5 mg/ml, but in general ultrafiltrates are required. The validity of the procedure was tested by determinations of the stability constants of inorganic phosphate and ADP with magnesium. The values obtained agree well with reported data.