Drainage of the partially wetting film dodecane on a vertical silicon substrate was investigated using image scanning ellipsometry (ISE). The ability of ISE to map every point on the surface simultaneously was demonstrated for the much more complicated case of the partially wetting fluid. Depending on the relative magnitude of the intermolecular, gravitational, and capillary forces, four flow regions were identified : the interfacial, transition, hydrodynamic, and meniscus regions. The partially wetting fluid began draining according to hydrodynamic theory, but after a short period the monotonically increasing film thickness profile became unstable and a mound or incipient drop began to form similar to 1 mm from the leading edge of the fluid. Although the mound grew over time, differential drainage was never strong enough to nucleate a dry patch, and so the film drained to an adsorbed layer. Two-dimensional profiles show the thickness profile first becoming unstable in the flow direction and then evolving a secondary instability perpendicular to the flow direction. The junction line between the interfacial and transition regions moved down the plate with an apparent diffusivity : D-cl = 4.9 x 10(-13) m(2)/s.