The adsorption of the n-alkanes, ranging from butane to dodecane, in the zeolites silicalite and (all silica) mordenite has been studied in the zero coverage limit using computer simulations. The heats of adsorption, Henry coefficients, and distribution of the n-alkanes over the different zeolite channels are calculated. The calculated heats of adsorption are in good agreement with available experimental data. The calculated heats of adsorption in silicalite show an unexpected alkane length dependence. The incremental heat of adsorption per carbon atom is larger for the longer alkanes than for the shorter alkanes. This observation is attributed to preferential adsorption of the longer alkanes in the straight channels of silicalite. Simulations of this type were made possible through the development of a novel Monte Carlo simulation technique which is orders of magnitude more efficient than the conventional simulation techniques.