If we study color reproduction, such as computer color matching or the appraisal of metametric index, we wish to understand the characteristic of color differences that are caused by the object spectral reflectivity change at each wavelength. If we simulate the light source, we wish to know the characteristics of color differences that are caused by change in relative power distribution of the light source at each wavelength; if we simulate a human eye instrument, we wish to know the characteristics of color differences that are caused by change in visual sense of human eyes at each wavelength. So, we define the color-sensitivity functions of an object, a light source, and human eyes. According to the chromatic theory, the color-sensitive Junctions of an object, a light source, and human eyes are defined in the widely used CIE1976 (L*a*b*) color space and color difference.(1) Their mathematical formulae are deduced. The three kinds of color-sensitive functions are studied systematically and comprehensively in the whole color space. The characteristics of the color-sensitive functions are summarized, and the mathematical models of the three kinds of color-sensitive functions can be utilized in some fields such as computer color matching, simulation of a standard light source, and humans viewing a colorimeter. (C) 2005 Wiley Periodicals. Inc. Col Res Appl, 30, 118-124, 2005; Published online in Wiley InterScience (www.interscience.wiley.com).