The in-situ, macroscopic dynamics of diffusion and gel growth during phase inversion of polyethersulfone (PES) solutions have been investigated using optical monitoring of the dark-ground and reflected light images which form on solution-nonsolvent contact. Data are reported for various ternary PES/solvent/nonsolvent systems. Analysis of two molecular weight systems shows that, under rapid quench conditions, a well-defined liquid demixing zone forms ahead of the gelation zone. Related to this, the occurrence of an initial region of anomalously high front motion followed by a more mild t(1/2) behavior suggests a dependence of the system diffusive transport properties on the relative time scales for gel formation and nonsolvent diffusion. The dependencies of the gel and diffusion front motions on solution concentration and nonsolvent solubility parameter are quantified in terms of the slope of the linear t(1/2) portion of the associated time-dependent curve. Rationales based on mass transfer models for both delayed and rapid precipitation systems support the concept of quantifying front motions in terms of an effective diffusion coefficient, D-c, for either the homogeneous diffusion or gelation dynamics.