Recent data has shown that the process of chromatic adaptation might be asymmetrical, or irreversible, and that this effect might be more than simply a manifestation of the time course of adaptation. This paper introduces a simple modification of the von Kries chromatic adaptation transform, referred to as vK20, that can account for the asymmetry in chromatic adaptation through inclusion of previous adapting conditions. Also introduced is a new reference chromaticity (∼15000K) for degree of adaptation that seems more physiologically plausible than the commonly used equal-energy (EE) illuminant or CIE illuminant D65.