A set of simple equations for the calculation of tristimulus values of subtractive color mixtures in solution has been developed for the specialized case in which the ratios of transmittances of the component colorants are constant over all wave-lengths in the visible spectrum. The values calculated are shown to represent the maximum values for mixtures of components with ratios of transmittance which differ from the specialized case, and it may also be demonstrated that no points of inflection may exist in the plots of tristimulus values vs. concentration of components. In practical mixtures, departures from these theoretically derived relationships are due to interaction of the components. In use, the equations prove to be sufficiently accurate for calculation of tristimulus values of most mixtures in which the color gamut is small.