An empirical adsorption equation is presented for predicting the adsorption isotherms from aqueous solution for a wide variety of hydrophobic porous adsorbent—adsorbate systems. The equation, which is based on the multiple linear regression analysis, comprises four parameters: surface area ( S), mean pore diameter ( D), molecular refraction ( M r ), and number of hydrophilic functional group ( N f ). Mr of adsorbate offers a basis for expressing quantitatively the adsorbent—adsorbate interactions associated with the dispersion forces. N f in an adsorbate molecule represents the strength of the adsorbate—water interactions. S and D of adsorbent represent the adsorptive capacity and the adsorptive strength, respectively.