In situ C-13 CP-MAS NMR measurements enabled for the first time detection of the presence and absence of water liquid phase in nanospaces of alkyl-grafted mesoporous silicas through the signals of alkyl chains, when the materials were immersed in water It was revealed that n-octyl-grafted mesoporous silica excluded liquid water from the hydrophobic nanospaces at least in an hour time scale while liquid water penetrated into n-pentyl-grafted material much faster This is important for understanding the adsorbents in the working states in water