Kinetic adsorption isotherms were obtained by gravimetric determination of water adsorption into fully hydroxylated mesoporous silicas using samples exposed to controlled humidity air at 22+/-2 degrees C. Twenty kinetic isotherms at several relative humidities (11, 33, 43, 51, 75, and 85%) were obtained with 11 different batches of silica using this simple procedure to obtain quantitative information on the formation of H2O adsorbates. The H2O surface concentrations obtained from the plateau data of individual kinetic adsorption isotherms at 43 and 51% RH, typically precise to about +/-1%, show that a complete monolayer is formed with negligible second-layer adsorption at these relative humidities. This monolayer has a surface concentration of 7.68+/-0.30 micromol H2O/m2, which is lower than the quasi-equilibrium concentration at these relative humidities obtained by the conventional equilibrium-isotherm procedure. Comparison with the Kiselev-Zhuravlev concentration of silanol groups on fully hydroxylated silicas (7.6+/-0.8 micromol SiOH/m2) confirms 1:1 H2O:SiOH stoichiometry of this monolayer. The presence of partial-layer structures at 2.85+/-0.1 and 5.7+/-0.1 micromol H2O/m2 is suggested by isotherms at 11 and 33% RH, respectively, while a bilayer at approximately 14+/-1 micromol H2O/m2 is suggested by kinetic isotherms at 75 and 85% RH.