We report the fabrication of microporous thin film membranes with two-dimensionally arranged submicron pores whose size can be varied by changing pH of aqueous medium. A solution containing poly(2-vinylpyridine) partially quaternized with 1,4-diiodobutane (qP2VP) and unreacted 1,4-diodobutane (DIB) was used for the formation and deposition of the membranes on solid substrates. The membranes were spin-coated onto solid substrates in a controlled humid environment. The presence of water vapor in air was found to be a necessary condition for the pore formation. We studied the influence of relative humidity on the membrane morphology and proposed a mechanism of pore formation. Cross-linking the qP2VP membranes with DIB made them insoluble (stable) in organic solvents and acidic water. The cross-linked membranes demonstrated pH-dependent swelling, which had a strong influence on the pore size.