A novel method for preparing silica membranes with controlled pore size and porosity is reported. Mesoporous silica membranes were modified at near room temperature, using catalyzed atomic layer deposition (C-ALD) of silicon dioxide within the membrane. The catalyst, pyridine, in addition to reducing the reaction temperature, acted as a template for controlling pore size. The catalyst limited the deposition reaction within the mesoporous matrix, by reactant exclusion from the pores. This self-limiting pore size reduction was confirmed using single-gas permeation measurements on the modified membranes. The self-limiting pore size reduction was found extremely beneficial for reducing defects which contributed to viscous flow in the mesoporous membrane, without plugging up the membrane and maintaining reasonable fluxes. A mesoporous membrane with a N-2 permeance of 3.5 x 10(-7) mol(.)m(-2.)s(-1.)Pa(-1), 3.3% of the flux of which was attributed to viscous flow, had its viscous flow reduced to less than 1% with a modest N-2 permeance reduction to 2.8 x 10(-7) mol(.)m(-2.)s(-1.)Pa(-1). Separation experiments with permanent gases indicated the pore size was in the transition region between mesoporous and microporous. A modest separation between CH4 and H-2 of 8.6 was observed at 473 K for a modified membrane. The separation of p- over o-xylene was improved from 1 to 2.1 after modification of the mesoporous membrane with C-ALD. The permeance of p-xylene was 2.5 x 10(-7) mol(.)m(-2.)s(-1.)Pa(-1). This novel synthesis method for membranes has the potential for pore size control in the transition region between microporous and mesoporous, near 1 nm, by using appropriate catalysts for the C-ALD process. In addition, it has potential as a general method for defect repair in inorganic membranes.