The exceptional hydrothermal- and solvent stability of organically linked silica makes it a promising candidate material for molecular separation membranes. Tailoring towards specific separation properties however requires precise control over the pore structure. Here we show that this can be achieved by adjusting the acid-to-Si ratio immediately before the onset of physical drying of the 1,2-bis(triethoxysilyl)ethane-based polymeric colloidal sols. This procedure provides uniform coating conditions and results in defect-free films. The structure development is investigated with in-situ Small-Angle X-ray Scattering, both in a solvent and during film drying. Acid-catalyzed colloidal growth in a solvent is governed by diffusion-limited cluster aggregation for all studied acid concentrations. Upon solvent evaporation, micropores (<2 nm) are formed at low acid concentrations (H+:Si ⩽ 0.1). This can be explained by compressive capillary forces exerted by the receding pore liquid. At H+:Si = 1, reaction-limited cluster aggregation is observed during drying, followed by the formation of pores >2 nm. The compressive forces are balanced by a network strengthened by ongoing condensation reactions, and by the positive charge on the hybrid organosilica. This results in a larger pore size under more acidic conditions. The permselectivity of the supported membranes correlates with the pore structure of the unsupported materials. Thus, the adaptability of the pore structure allows a wider applicability of organosilica membranes in energy-efficient industrial molecular separations.