In a thin film blend of deuterated poly(methyl methacrylate) (dPMMA) and a random copolymer of poly(styrene-co-acrylonitrile) at the critical composition, the dPMMA-rich phase wets the surface while the underlying layer undergoes phase separation. Using forward recoil spectrometry, the figure shows that the thickness of the surface layer grows linearly with time, consistent with hydrodynamic-flow driven wetting. Using scanning force microscopy (SFM), surface roughness is found increasing with time. After selectively etching the dPMMA-rich phase to expose the non-wet SAN-rich morphology, SFM shows that just below the wet layer lies a bicontinuous dPMMA and SAN-rich interconnected network. The dPMMA-rich channels provide a pathway for rapid dPMMA transport to the surface. Taken together, the observation of fast wetting layer growth, t^1, and connectivity between the wetting phase and internal morphology provides excellent support for hydrodynamic-flow driven wetting of the dPMMA-rich phase.