An adaptation of Kubelka's general model of diffuse reflectance and diffuse transmittance of light to nonideal scattering samples is proposed. It is applied to quant. Fourier transform IR spectrometry of nondiluted surface-derivatized silica powders. Corrections of the measurements are introduced in order to take into account common problems that often prevent a truly quant. application of the DRIFTS anal. method. Effects on the measured data due to the background absorption of the sample and to the specular reflection on its surface are discussed and quant. cor. for. The possible existence of gradients of the optical properties of the powder medium is taken into account by a numerical adaptation of the model to inhomogeneities of the phenomenol. absorption and scattering coeffs. K and S. Model systems constituted of two types of silica powders of very different morphol. are prepd. Known concns. of mols. carrying a cyano functional group as chromophore were covalently anchored on the powder surface. Application of the adapted model gives a quite good description of the measured diffuse reflectance and diffuse transmittance in these cases where the simple Kubelka-Munk function fails.