Monitoring and controlling structure evolution of nanoporous materials during their synthesis or modification can be achieved by innovative combinations of in-situ and ex-situ techniques applied in consecutive or real-time modes. In the present study, tailoring of the pore size of silica membranes was achieved by applying combinations of in-situ gas permeability with mercury porosimetry, SEM and cross sectional EDAX, during the membrane modification using two CVD modes: in the normal cyclic mode, a sequential exposure to silica precursor vapors and O3 was performed whereas in the pressure-pulsed mode, the exposure to silica precursor was accompanied by pressure pulses of the order of 400kPa. The membranes were treated with tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS) in both modes for variant CVD durations. The aim was to examine effects in the pore-narrowing rate, deposition profiles and membrane performance characteristics that are induced due to the different physicochemical properties of the involved precursors, as well as due to the different CVD conditions. A common observation for all cases studied was that the He/N2 permselectivity at 573K, starting from 15, presents a maximum value of about 100 when interpreted as a function of the organosilane exposure period and then drops upon further treatment. The fact that this maximum appeared at shorter exposure periods in the case of the TMOS treated membranes and especially in the case of the pressure-pulsed mode reveals the increased degree of penetration of the smaller tetramethoxysilane molecule inside the surface pores of the separation layer, the faster rate of its polymerization on the pore surface and the higher efficiency of the pressure-pulsed mode. Furthermore, cross sectional EDAX analysis in conjunction with mercury intrusion was applied to determine the profiles of silica deposition and elucidate the origin of the different pore blocking rates and corresponding membrane performance.