The use of surface pressure as a variable to describe adsorption made possible the evaluation of the thermodynamic equilibrium constant, the standard free energy, and enthalpy of the adsorption reaction of phosphate on hydrous Al oxide. The values obtained at 299 degrees K are as follows : K = 1.567 x 10(-2) cm, Delta G degrees = 10.33 kJ mol(-1), and Delta H degrees = 25.11 kJ mol(-1). The magnitude of the values as well as the values of the isosteric heat of the reaction (-23.7 to -29.4 kJ mol(-1)) are of the order expected for a ligand exchange reaction on an hydroxylated heterogeneous surface.