Microporous aluminum methylphosphonates, AlMepO-alpha and -beta, prepared by different procedures, were characterized mainly using Al-27, (31)p, and C-13 MAS NMR, TG-DTA, and IR, The MAS MMR spectra were consistent with the crystal structures determined by the single-crystal X-ray structural analysis published previously. All the P-31 NMR signals were reasonably assigned using an assumed correlation between P-31 chemical shift and the mean Al-O-P angle around the phosphorus sites. Nitrogen adsorption isotherms of AlMepO-beta degassed at elevated temperatures were of type I, while those of AlMepO-alpha gave two plateaus in the low relative pressure region, The stepwise adsorption was explained by a packing change of the adsorbate on adsorption. The pore diameter calculated from the maximum nitrogen adsorption capacity was consistent with adsorption of 2,2-dimethylpropane but was larger than the size expected from the crystal structure of both the compounds, The water vapor isotherm was type II in the low relative pressure region, confirming the hydrophobic nature for both compounds, AlMepO-alpha indicated no apparent adsorption of water into the channel, but AlMepO-beta showed a sudden adsorption of water at P/P-0 approximate to 0.7. The difference in the water vapor isotherms between the compounds was explained based on the relationship of the size of the water clusters formed and the shape of the adsorbent channel.