The structure of solid methylalumoxane (MAO) and its behavior in the temperature interval from 20 to 250 degreesC have been studied using IR spectroscopy in diffusion reflection mode (DRIFT) and mass-spectrometric (MS) methods. It has been shown that three-dimensional MAO molecule may entrap water molecule in its volume (absorption bands (a.b.) 3550 cm(-1) in the IR-spectrum). Using a density functional theory (DFT) quantum-chemical method, the three-dimensional molecular model of MAO with composition (-Al(CH3)O-)(12), comprising water molecule in its structure, has been calculated. The release of the water molecule from the structure of MAO molecule is accompanied by the protolysis of Al-CH3 bonds and methane evolving. This reaction seems to be responsible for the observed evolving of methane on the MAO aging. As the temperature of MAO heating increases from 20 to 150 degreesC, the process of methane evolving intensifies. The heating of the solid MAO samples is also accompanied by the release of trimethylaluminum, (TMA) in significant amount (0.18 mole AlMe3/mole Al-MAO at 100 degreesC. Obviously, the released TMA was strongly associated with the MAO in the initial sample. The data on the activity in the ethylene polymerization of catalyst Cp2ZrCl2/MAO prepared with the use of the MAO samples dried at temperatures 20-250 degreesC are presented as well.