Small-angle light scattering and scanning electron microscopy (SEM) have been used to study morphology development in poly(methyl methacrylate) membranes formed by the thermal inversion method. Analysis of the early time behavior of the scattered intensity in terms of Cahn-Hilliard theory establishes spinodal decomposition as the mechanism of phase separation for the conditions studied. The intensity maximum position for late-stage growth follows a power law dependence with a time exponent of approximately one-third, which is consistent with diffusive coarsening mechanisms. Solutions quenched to regions of the phase diagram below the glass transition temperature show an immediate arrest in the location of the scattered intensity maximum, indicating a locking-in of the phase-separated structure. Domain sizes obtained from SEM measurements of annealed, quenched samples are shown to be consistent with light scattering measurements.