Abstract: Long-column method was used to determine the adsorption isotherms of 4 VOCs (benzene, toluene, chloroform and acetone) in concentration range of 250～5000mg·m^-3 on a commercial activated-carbon under different humidity levels at 30℃.A modified Polanyi-Dubinin equation was proposed to correlate the adsorption equilibrium of different VOC-water vapor systems. Among 3 methods of calculating the Relative Affinity Coefficient β used,the Molar Volume method and the Molecular Parachor method proved to be suitable for the calculation with better precision than the Electronic Polarization method. Calculation results were satisfactory for the benzene-, toluene-, and chloroform-water vapor/activated carbon systems, but poor for acetone possibly because of its strong polarity.The equation could be used to estimate the detaining effect of atmospheric humidity on the adsorption equilibrium of VOCs on activated carbon.