Water uptake at 298 K by two mesoporous silicas with different pore sizes was studied using volumetric vapor sorption. Through variation of sample pretreatment temperature and time, the number of surface hydroxyl groups was varied, leading to marked changes in the water sorption behavior. The BET model was used to measure surface hydroxyl density from low pressure parts of water adsorption isotherms. An adsorbed phase model is utilized to calculate, for the first time, the distribution of pore water molecules between adsorbed and pore condensation phases and characterize the density and thickness of the water sorption phase as functions of surface hydroxylation and pore size. With increasing surface hydroxyl density, the adsorption of water to the pore surfaces increases, leading to formation of thicker and denser water sorption layers. Monolayer coverage is reached at reduced pressure of ca. 0.3. The onset of adsorption pore condensation of water shifts to lower reduced pressure with increasing surface hydroxyl density, indicating growing thickness of adsorption layers. However, the water pore condensation step of the desorption branch shifts to smaller reduced pressure with increasing surface hydroxylation, reducing the adsorption–desorption hysteresis width. In the smaller pores, the adsorbed phase forms a sparse monolayer, while approximately a bilayer is formed in the wider pores. The previously reported silica nanopore underfilling by water is confirmed and rationalized by a significantly reduced water density in the adsorbed phase with respect to bulk, while the pore core is filled with water of approximately bulk density.