Hybrid organosilica membranes have become attractive for industrial applications because of high performance and long-term stability. This work investigated the influence of water vapor on CO2 gas permeation through the hybrid membranes. Two types of organoalkoxysilanes, bis(triethoxysilyl)ethane (BTESE) and bis(triethoxysilyl)octane (BTESO), were used as precursors to prepare membranes via the sol-gel method. The two membranes showed distinct properties of porosity and water affinity because of the differences in the bridging methylene numbers between the two Si atoms. Under dry conditions, the BTESE and BTESO membranes showed CO2 permeances as high as 7.66 x 10(-7) and 6.63 x 10(-7) mol m(-2) S-1 Pa-(1) with CO2/N-2 selectivities of 36.1 and 12.6 at 40 degrees C, respectively. In the presence of water vapor, CO2 permeance was decreased for both membranes, but the effect of water vapor on CO2 permeation was slighter for BTESO membranes than it was for BTESE membranes because of more hydrophobicity and denser structures with a longer linking-bridge group. The hybrid organosilica membranes both showed good reproducibility and stability in water vapor.