A comparative light transmission and scanning electron microscopy study of the kinetics of pore structure formation for membranes based on polysulfone (PS), polycaproamide (PCA), and polyvinylidene fluoride (PVDF) from solutions in DMAA has been performed. It has been shown that the structure formation rate is determined by the diffusion mechanism for PVDF and the interplay of spinodal decomposition, convection, and diffusion for PS, whereas phase separation occurs in the case of PCA with a delay due to the high viscosity and the surface tension of the solutions.