The apparent binding constant (K_app) of calcium (II) with 2, 7-bis(4'-chloro-2'-phosphonophenylazo)-1, 8-dihydroxynapthalene-3, 6-disulfonic acid (chlorophos-phonazo III) was determined in the pH range of 6.08.0 by spectrophotometry at 20°C. In the presence of 0.1 M KCl, K_app increased by a factor of 50 between pH 6.0 and 8.0. However, the pH dependency of K_app above pH 7 was slightly smaller than that of K_app below pH 7. At pH 6.8 K_app was 1×10⁶ M^-1. The present data may be available to measure free calcium (II) concentration in biochemical experiment.