The gas-adsorption behavior in microporous transition-metal complexes was investigated by the use of Monte Carlo simulations. We prepared the models of Cu3TMA2and Zn(BDC), which were crystallographically determined. The pressure dependence of the amount of adsorbed gases (argon, nitrogen, and hydrogen) was simulated at the boiling temperature of the gases. In the gas adsorption in Cu3TMA2, the effect of the charge distribution on the framework is negligible. However, the coordinating water narrows the cavity and stabilizes the adsorbed argon molecules. The amounts of adsorbed gases and the isosteric heats were calculated for Cu3TMA2and Zn(BDC) and the pictures of probability distribution of gases in the cavity were obtained. The probability distributions indicated that the adsorbed gases tend to be distributed as a zig-zag column in the capillary of Zn(BDC) and as a shape like a four-leaf clover avoiding the four benzene rings in the narrow cavity of Cu3TMA2.