In this study, a new method is introduced for the determination of dye concentration in fabrics dyed with bicomponent dye mixtures. The reflectance spectra of the samples dyed with different binary mixtures of dyes Were recorded between 400 and 700 nm. The obtained (K/S) spectra were divided by a standard (K/S) spectrum of each (K/S) the components in the mixtures and the derivative spectra were calculated. The amounts of dyes were determined by measurements in suitably selected wavelengths in the acquired derivative ratio spectra. The obtained results indicate that the developed derivative ratio (K/S) spectra method is more accurate than the normal Kubelka-Munk method. The proposed derivative method is simple, accurate, and suitable for quantitative analysis of samples dyed with binary mixed shades. (C) 2010 Wiley Periodicals, Inc. Col Res Appl. 35. 193-199. 2010: Published online 14 January 2010 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20568