Porous poly(oxymethylene) membranes were prepared as now solvent-resistant membranes by a thermally induced phase-separation method. Porous structures were formed by solid-liquid phase separation (polymer crystallization) rather than liquid-liquid phase separation. The pores existed in the intraspherulitic and interspherulitic regions. The effects of the polymer weight percentage and cooling rate on toluene permeance and solute rejection were investigated. The solvent resistance of the membranes was tested by the immersion of the membranes in organic solvents for 1 month, and high durability against the solvents was confirmed.