The phase separation of isopycnic polystyrene-diethyl malonate solutions has been studied by investigating the microstructure of polymer membranes. Polymer solutions underwent spinodal decomposition and coarsening via a thermally induced phase separation-procedure, and supercritical CO2 extraction was employed to remove solvent, resulting in microporous membranes. At relatively short Coarsening times, the coarsening rate of the cell size can be expressed as a power law in time with the exponent increasing with increasing quench depth; for deep quenches, the growth rate has an exponent of 1/3 in agreement with the classic theories for coarsening by Ostwald ripening or coalescence. At longer coarsening times, there was a crossover to a much faster growth rate, yielding an exponent of 1.0 independent of phase separation temperature, consistent with expectations for the hydrodynamic flow mechanism of coarsening suggested by Siggia. This is the first experimental Confirmation of the evolution of the coarsening mechanism from one mechanism with a growth rate consistent with Ostwald ripening or coalescence to a second mechanism with a faster growth rate. Comparisons were also made to coarsening observed-in nonisopycnic/low-viscosity polystyrene-cyclohexane systems where strong gravitational effects dominate the phase separation process : at relatively short times and crossover effects cannot be observed.