The solubility of 1-hydroxyethane-1,1-diphosphonic acid (HEDP) monohydrate (A) in binary acetic acid (B) + water (C) solvent mixtures was measured by using the isothermal method from (293.15 to 328.15) K. A laser monitoring observation technique was employed to determine the dissolution of the solid phase in a solid + liquid mixture. The solubility data were correlated by the combined nearly ideal binary solvent (CNIBS)/Redlich-Kister equation.