Adsorption of alkanes, aromatics, and other organic molecules in liquid phase and the dense vapor phase was studied on FAU and MFI zeolites at temperatures ranging from 25 to 300 degrees C. On faujasites, no competitive adsorption between n-alkanes of different carbon number was observed at room temperature. At higher temperatures and lower fluid densities, longer n-alkanes start being adsorbed preferentially over short n-alkanes on faujasites. On MFI zeolites, longer n-alkanes have larger partition coefficients than short n-alkanes. The difference in competitive adsorption behavior resides in the different pore structures of these zeolites : mutual interactions of several molecules adsorbed in the supercages of the faujasites dominate over direct interaction with the zeolites＇ force field, while on MFI zeolites, only one n-alkane molecule fits in the cross diameter of the pore, thereby strongly reducing the adsorbate-adsorbate interactions. On MFI zeolites, small n-alkanes have unrestricted access to both channel types, while long n-alkanes have restricted access to part of the micropore structure. Separation by selective adsorption in liquid phase on faujasites depends to a high extent on differences in building blocks of the molecules.