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A setup for surface-plasmon-resonance- (SPR) based imaging ellipsometry was developed, which gains from the sensitivities of both SPR and ellipsometry to ultrathin film parameters. It is based on Otto's configuration for prism-sample coupling and a wide-beam imaging ellipsometry. A set of ultrathin gold and silver films was measured to determine their optical constants and thicknesses. Coupling the sample using a prism with a convex surface enables us to capture images of generated SPR elliptical fringes, which correspond to different SPR amplitude values at different air gap thicknesses. Analysis of the images acquired at different polarizer and analyzer angles provides the ellipsometric functions Ψ and Δ versus thickness of air gap and hence the extraction of the optical constants of ultrathin metal films. The measured film thickness is in agreement with the results of x-ray reflectivity measurements.