The solubility product of Cd5H2(PO4)(4). 4H(2)O at 37 degreesC has been determined by mixing dilute solutions of cadmium nitrate and ammonium phosphate and measuring cadmium and phosphate concentrations as well as the pH values of the saturated solutions. Using the definition K-sp = a(Cd2+)(5)a(HPO42-)(4)/a(H+)(2), the value pK(sp) = (30.9 +/-0.3) was found. Necessary values of stability constants of cadmium phosphate complexes in solution were estimated from literature values determined at high ionic strength with the aid of mean ion activity coefficients from independent sources.