Xe-129 NMR spectra of xenon dissolved in acetonitrile confined into three mesoporous silica eels with nominal pore diameters of 40, 60, and 100 A have been measured over the temperature range 170-245 K. The spectra consist of a number of lines. which contain detailed information on the system. The most interesting result is that the chemical shift of a particular signal observed below the melting point of confined acetonitrile is highly sensitive to the pore size, and hence its shape is sensitive to the pore size distribution function. This signal originates from the xenon atoms sited in very small cavities built up inside the pores during the freezing transition. It can be used to determine the size or even the size distribution function of the pores. In addition. the emergence of this signal reveals the phase transition temperature of acetonitrile inside the pores. which can also be used to determine the size of the pores. The difference in the chemical shifts of two other signals. which arise from xenon dissolved in bulk and confined acetonitrile, provides still another novel method for determining the size of the pores.