The sorption of xylene and isooctane was measured by the gravimetric flow sorption method on commercial activated carbons (ACs), namely, virgin and reactivated ACs. Nitrogen physisorption, high-pressure mercury porosimetry, helium pycnometry, iodine number, pH measurement, Raman and X-ray photoelectron spectroscopies were applied for textural, structural, and surface characterization of ACs. The equilibrium geometries of isooctane and xylene molecules were modeled using density functional theory (DFT) calculations. Their dimensions were estimated to be correlated with textural properties of ACs to reveal the effect of size selectivity. The key factors influencing the adsorption capacity of the chosen ACs for both tested volatile organic compounds (VOCs) were found to be the mesopore surface area and the surface basicity.