The adsorption of a purified solution of octanoic acid at the mercury/solution and air/water interfaces was studied. The equilibrium capacitance values were obtained by phase sensitive ac voltametry, From the concentration dependence of the capacitance and of the surface tension at 22 and 45 degrees C, the adsorption isotherms were evaluated by applying the Frumkin isotherm, Adsorption energies, lateral interaction energies, and molecular surface areas obtained are compared for the two interfaces.