Membrane formation by nonsolvent induced phase inversion was directly observed using light microscopy. Polysulfone (PSf) was used as a model polymer while 1-methyl-2-pyrrolidinone (NMP), N,N-dimethylformamide (DMF), water and glycerol were used as model solvents and nonsolvents. Direct observation and kinetic analyses suggest finger-like macrovoids formed by convective flow of nonsolvent into the polymersolvent solution; convective flow arose from interfacial energy gradients at the polymer solutionnonsolvent interface. Convective nonsolvent flow into the polymer solution was hindered by the formation of surface skin layers or viscous gel layers within the polymer solution film. Viscous gel layers were often formed when using a poor nonsolvent, poor solvent, or an insufficient supply of a good nonsolvent. Greater viscous hindrance resulted in membranes with shorter or no finger-like macrovoids. Large finger-like macrovoids propagated only when nonsolvent convective flows into the polymer solution film exceeded viscous hindrance forces, i.e., when a good solvent, ample supply of good nonsolvent, and polymer solution of relatively lower viscosity were used.