Raman spectroscopy is used as an in situ diagnostic to measure the oxidation of ethanol by oxygen in supercritical water. An elementary reaction mechanism based on the work of Marinov is shown to predict accurately many of the experimental observations. Experimental measurements are reported at 24.5 MPa over a temperature range of 410-470 degreesC in supercritical water with reaction times ranging from 0.5 to 3.0 s. Concentrations of ethanol, acetaldehyde, formaldehyde, methanol, carbon monoxide, carbon dioxide, and hydrogen peroxide are measured as functions of time and temperature. The data show that the formaldehyde is the primary stable organic intermediate. An elementary reaction mechanism, modified for supercritical water conditions and supplemented with key methylperoxyl reactions, is used to interpret the observations. The experimental data are consistent with the purely radical chain oxidation process represented by this mechanism. Analysis of the mechanism identifies the primary oxidation pathway proceeding through acetaldehyde with oxidation routes involving initial abstraction of the hydroxyl hydrogen or a hydrogen atom from the secondary carbon. A pathway originating from H-abstraction from the methyl group of the ethanol molecule contributes to the overall conversion to a lesser degree.