A new technique is presented for determination of the diffusion behavior of dyes in fibrous materials. Polyamide 6.6 in filament form has been dyed with an acid dye in different temperatures and times. The microscopic images of the cross sections of dyed samples are captured and the RGB color images of the surfaces are converted to gray scale intensity data. Then, the variations of intensities within the fiber diameter, from the surface to the center of fiber, are analyzed. A linear relation is assumed between the concentration and the intensity for each pixel. The concentration-distance curves are plotted and the pseudo-diffusion coefficient of dye is predicted by using the error function relation between the normalized intensity and diffusion coefficient. According to the results, the index is constant for the samples dyed in different times and changes significantly by temperature. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1030-1036, 2009