Crude oil incompatibility is a common problem in the oil industry and may lead to severe fouling of process units, such as the pre-distillation heat exchanger train. This work investigates the use of ellipsometry to evaluate the compatibility of crude oil blends. Two crude oils with predicted regions of compatibility and incompatibility were blended in several ratios and the formation of a denser phase was examined by ellipsometry. The procedure implies the evaluation of the real part of the refractive index of the crude oils and their mixtures. which can be linked to the density of the mixture. The ellipsometric measurements fit well theoretical predictions obtained using a Cauchy dielectric function for the pure crudes and the Maxwell-Garnett effective medium model for the mixtures. The regions of compatibility and incompatibility between the two crudes were checked against independent measurements of deposit formation by weight and predictions based on the insolubility and solubility blending numbers, showing good agreement. (C) 2011 Elsevier B.V. All rights reserved.