Dilute solutions of ethyl cellulose (ETC) in acetone and of poly(ethylene oxide) (PEO) in ethyl acetate, acetonitrile, ethyl acetate-acetonitrile, and acetonitrile-water mixtures were expanded isothermally by compressed CO2. Onset precipitation pressures were visually measured through a windowed cell.Toward a rational understanding of the molecular mechanisms involved in gas anti-solvent (GAS) processes, saturated-liquid-phase volume expansion and solvent power were monitored by UV-vis spectroscopy for the solvent mixtures considered in the precipitation experiments. Ferrocene absorbance and phenol blue absorption-peak-wavelengh shifts were used as probes to assess saturated-liquid-phase volume expansion and solvent power, respectively.For the first time, a correlation between a microscopic bulk property, solvent power, and the onset precipitation pressure of a solute is reported. Because of preferential interactions with the dye (hydrogen bonds), the correlation breaks down when even small amounts of water are present in the solvent mixture.The results presented here suggest that UV-vis spectroscopy constitutes a valuable tool for understanding some phenomena related to supercritical-fluid technology.