The results of extensive grand canonical Monte Carlo simulations of adsorption in siliceous zeolites are reported and used to assess the accuracy of a size-weighted (SW) combining rule for Lennard-Jones cross-parameters. Adsorption isotherms, Henry's Law constants, and isosteric heats were computed for 45 different systems consisting of unary, binary, ternary, and quaternary mixtures of light hydrocarbons in silicalite, pure methanol, and carbon dioxide in silicalite and light hydrocarbons in siliceous forms of the zeolites TON, LTA, and CHA. A comparison is made with the conventional Lorentz-Berthelot (LB) combining rule as well as other force fields and experimental data. In addition, the influence of intramolecular scaling methods and long-range tail corrections on computed properties was evaluated. Overall, the SW combining rule gives similar, and sometimes superior, results when compared to the LB combining rule as well as force fields tuned to experimental data.