Densities of pure 1,3-dimethylimidazolium methylsulfate ionic liquid [MMIM][CH3SO4] and its mixtures with methanol have been measured with an accuracy of +/- 0.2 kg center dot m(-3) over a temperature range from (313.15 to 333.15) K and a pressure range from (0.1 to 25) MPa using a vibrating tube densimeter. The isothermal compressibilities, isobaric expansivities, and excess volumes have been calculated. The uncertainties of these properties have been estimated to be +/- 0.05 GPa(-1), +/- 0.2 center dot 10(-1) K-1, and 0.1 cm(3 center dot)mol(-1). The exceptionally strong influence of pressure and temperature on the excess volumes has been observed.