The thermally grown InP oxide as etched by an aqueous dilute HF solution has been studied by ellipsometric techniques. The ex-situ measurement reveals a two-layer structure for the oxide grown at 440°C. The refractive indices for both oxide layers have been determined using a two-layer optical model. The etching process has also been monitored ellipsometrically in the real etching environment, in-situ. A fused silica cell, which enables the windows to be aligned properly, has been specifically designed for the in-situ solution measurement. A liquid layer at the solutionoxide interface has been identified, and the layer is shown to contain P and In species resulting from the etching reactions. A theory based on the Lorentz-Lorenz relation results in a reasonable qualitative description of the liquid layer. During the etching of the oxide the liquid layer shrinks at a linear rate, and after removal of the outer oxide layer the liquid layer forms a dense electric double layer
