R. O. Brown and D. I. MacLeod (1997) observed that chromatic patches appear much more saturated against an equiluminant, uniform gray surround than against a chromatically variegated surround with the same space-average color. Using asymmetric color matching, we investigated what stimulus conditions are critical for the occurrence of this "gamut expansion effect." We found (a) that the effect diminishes rapidly with increasing color contrast between target and surround, (b) that the amount and the spatial distribution of color variance in the surround plays but a very limited role, (c) that the effect is mainly local, and (d) that basically the same effect can be obtained by comparing two uniform surrounds. These findings, particularly the latter, argue strongly against an explanation solely in terms of contrast adaptation. We suggest that the main features of our findings can be explained in terms of color scission.