The porous structure of silica films obtained by monosilane oxidation by oxygen in an isothermal reactor in the temperature ran 150-250 degrees C and pressure 0.5-1.2 Torr has been investigated by means of adsorption porometry. It has been found that at T< 200 degrees C the minimum radius of registered pores increases sharply when the pressure in the reactor is : Increased. It is assumed, on the basis of experimental data, that tile elevation of pressure causes the increase in size of polymeric particles formed in the gas phase (clusters) that determine the size of the formed pores. At T > 200 degrees C, the minimum pore radius in the films is weakly dependent on pressure; besides, the film density increases during its growth due to solid-phase reactions within tile bulk of the film. The mechanisms of the influence of gas-phase stages of monosilane oxidation on the regularities of mesopore formation at deposition temperature within 150-200 degrees C is discussed, as well as the possibility of obtaining layers with a required porosity.