Absolute diffuse reflectance and transmittance FT-IR spectra of optically thin layers of surface-derivatized silica powders are measured, using a photopyroelectric detector placed in direct contact with the sample. The role of inhomogeneities in the surface of the powder for quantitativediffuse reflectance and transmittance measurements is examined. It is found to be equally important and nonnegligible for both sampling methods. Diffuse transmittance is shown to be more sensitive to changes of the depth of the layer than diffuse reflectance. The effect of the pressure applied to the sample surface on the scatteringcoefficient of two types of silica powders of different morphologies is demonstrated. A new analytical procedure based on DRIm spectrometry is applied to the quantitative study of the influence of the chemical environment of a cyano functional group covalently linked to the silica surface on its molar extinction coefficient.