The utility of the Kubelka-Munk equation for determining the fixation of reactive dyes to cotton has beet? assessed. Large K/S values are not linearly related to dye concentration on fabric and thus give an inaccurate assessment of fixed dye. If lower values of K/S are selected, by measuring at a suitable wavelength, a linear relationship exists with the concentration of dye present, even at very heavy depths of shade. Thus, by judicious choice of wavelength, K/S values can provide an accurate and convenient method for the assessment of dye on fabric, provided that a reference fabric sample, with a known concentration of the same dye, is available as a standard for comparison. Significant implications are then revealed for construction scalable models of opaque medium reflectance that are homogeneously linear over wavelength with respect to colorant concentration change. (c) 2006 Wiley Periodicals. Inc.