In situ X-ray diffraction measurements have been made on monolayer and capillary condensed acetonitrile in mesoporous MCM-41 C18 (pore diameter 37Å) in a temperature range of 298–200K. FTIR spectra of acetonitrile in C18 were also measured at 298K as a function of the vapor pressure of acetonitrile to confirm the monolayer and capillary-condensed acetonitrile in C18 used for the X-ray experiments. The radial distribution functions of the monolayer acetonitrile have shown that an acetonitrile molecule is hydrogen bonded via N atom to the silanol O atom with an N–O distance of ~2.6Å. The structure of the monolayer acetonitrile is almost independent of temperature, suggesting the strong hydrogen NO hydrogen bonds. On the other hand, the radial distribution functions of the capillary-condensed acetonitrile have shown that the structure of acetonitrile in the central region of the pores is gradually enhanced with lowering temperature, practically similar to that in the bulk. The present static structure of confined acetonitrile at low temperatures is compared with those predicted from molecular simulations and discussed with the dynamical properties previously obtained from quasi-elastic neutron scattering measurements.