Pale coatings of amorphous silica, sulfur, and sulfur-bearing compounds can form on terrestrial basalt flows while they are still active and subsequently on timescales ranging from days to years after the eruption. We cite examples below from Kilauea (Hawaii), Etna (Sicily) and Cerro Negro (Nicaragua) and caution against undue reliance on spectral reflectance to determine the bulk properties of lava flows on Io, where sulfur and/or sulfur compounds are likely to dominate comparable (but probably more widespread) coatings. ¿ American Geophysical Union 1996