The mesoporosity and microporosity of carbon aerogels were controlled by the structure of the polymer precursor and by CO2 activation, respectively. Their adsorption isotherms of nitrogen at 77 K were measured, and the porosities were determined by the high-resolution as-plot. The ratio of the micropore volume to the mesopore volume was 0.10 to 0.24. The adsorption isotherms of water on the carbon aerogels were also measured at 303 K. All water adsorption isotherms had an adsorption hysteresis near P/P-0 = 0.5. The hysteresis of the nonactivated sample is not marked, but activated samples showed a vertical and noticeable hysteresis. The saturated amounts of water adsorption were not close to the mesopore volume but the micropore volume. Hence, it was concluded that water molecules do not predominantly adsorb on carbon mesopores and that the adsorption hysteresis is not described by the Kelvin equation.