The degree of additivity-law failure of a mixture color consisting of two component chromatic colors has vend complex characteristics depending on the used component colors and their mixing conditions. It is significantly affected by, the mixture-color chromaticity and by the brightness/luminance ratio at the chromaticity,. A simple relationship was derived between the following quantities: degree of additivity-law failure, additivity-law luminance (luminance of the mixture color derived by postulating additivity, law), and brightness/luminance ratio at the chromaticity, of the mixture color. This relationship can be applied to any additivity-law failure experiment and any, formula on brightness/luminance ratio.