The discovery of SO2 gas in Io's atmosphere and possible frost on Io's surface prompted us to measure the spectral reflectance of laboratory SO2 front in the range 0.24--0.85 &mgr;m. The frost samples were grown in suitable vacuum at 130¿K and bi-directional reflectance spectra were obtained by illuminating at 20¿ and observing at 60¿ from the sample normal. The spectrum of typical SO2 frost has very low reflectivity (2--5%) at 0.30 &mgr;m, a steep absorption edge rising at 0.32 &mgr;m, maximum reflectivity (75--80%) at 0.4 &mgr;m, and uniformly high reflectance throughout the visible and near infrared. These results, when compared with Io's full-disk, Earth-based reflectance spectrum, suggest that SO2 frost is not the dominant component of Io's optical surface layer. Various calculated UV-visible spectra for mixtures of SO2 frost with other plausible surface components (sulfur allotropes and sodium sulfide) provide a good match to Io's spectrum and limit optically thick SO2 frost coverage to less than about 20% of Io's average projected surface area; this limit varies from a high of ~25% on the leading hemisphere to a low of ~10% on the sub-Jupiter hemisphere.