Spatially resolved deep-ultraviolet (UV) Raman spectroscopy was applied to solutions of CO<sub>2</sub> and H<sub>2</sub>O or D<sub>2</sub>O subject to a temperature gradient in a thermally regulated high-pressure concentric-tube Raman cell in an attempt to measure a Soret effect in the vicinity of the critical point of CO<sub>2</sub>. Although Raman spectra of solutions of CO<sub>2</sub> dissolved in D<sub>2</sub>O, at 10 MPa and temperatures near the critical point of CO<sub>2</sub>, had adequate signal-to-noise and spatial resolution to observe a Soret effect with a Soret coefficient with magnitude |<i>S</i><sub>T</sub>| > 0.03, no evidence for an effect of this size was obtained for applied temperature gradients up to 19 °C. In contrast, the concentration of CO<sub>2</sub> dissolved in H<sub>2</sub>O was shown to vary significantly across the temperature gradient when excess CO<sub>2</sub> was present, but the results could be explained simply by the variation in CO<sub>2</sub> solubility over the temperature range and not by kinetic factors. For mixtures of D<sub>2</sub>O dissolved in scCO<sub>2</sub> at 10 MPa and temperatures close to the critical point of CO<sub>2</sub>, the Raman peaks for D<sub>2</sub>O were too weak to measure with confidence even at the limit of D<sub>2</sub>O solubility.

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