Abstract Microscopic and macroscopic wettabilities of fine silica powder surfaces loaded with trimethyl-silyl groups (TMS) were studied. The modification was carried out by the chemical reaction of hex-amethyldisilazane (HMDS) molecules with surface silanols and the TMS coverage was determined by elemental analysis of carbon. The microscopic wettability was investigated by water vapor adsorption and IR spectroscopic methods. The macroscopic wettability was examined by various preferential dispersion tests. The results are summarized as follows. (1) HMDS molecules preferentially reacted with free type surface silanols. (2) The macroscopic wettability drastically changed at a TMS coverage of about 40%. (3) The effect of steric hindrance appeared clearly at a TMS coverage above 40%. Hence, a part of the residual surface silanols is shielded by TMS and water molecules will not access these sites. (4) In the case of TMS coverage below about 40%, cooperative adsorption takes place at the first stage of adsorption. Then by the occurrence of multilayer adsorption, a continuous two-dimensional water layer is formed at the latter stage of adsorption. On the other hand, in the case of coverage above about 40%, the adsorbed amount was about 65% of the theoretical monolayer capacity in spite of a high relative pressure at about P/P 0 = 1. Therefore, such a continuous two-dimensional water layer is not formed on account of the steric hindrance of the TMS and multilayer adsorption will not occur in this case. The surface property is estimated to be hydrophobic. (5) The results of preferential dispersion tests as the macroscopic wettability are in agreement with the results of water vapor affinity as the microscopic wettability.