Display omitted •Atmospheric-prresure plasma was applied to the surface of organosilica membranes.•The pore size decreased and affinity increased with increasing plasma exposure time.•The permselectivity increased with increasing plasma exposure time.•The balance between the diffusion barrier and adsorption determined PV performance. The effects of the plasma treatment time on the gas permeation and pervaporation dehydration properties of organosilica membranes modified by atmospheric-pressure plasma were investigated. The He/N2 permeance ratio increased from 10 to over 100 as the plasma modification time increased, indicating that the pore size decreased by plasma modification. As measures of the pervaporation properties, the H2O permeance gradually increased, and the EtOH permeance gradually decreased with increasing plasma modification time, resulting in a higher H2O/EtOH permeance ratio from about 20 to over 1000. The enhanced separation performance is attributed to the densification of the pore structure with increasing plasma modification time. A comparison of the temperature dependence of the permeance for gas permeation and pervaporation dehydration indicated that plasma modification increased the affinity of the membrane matrix for water molecules. This enhanced permeation property can be attributed to the increased adsorption of water on the membrane surface with increasing plasma modification time. It was clarified that the permeation properties could be controlled through the tuning of the pore structure and water adsorption properties of the membranes by varying the plasma treatment time.