The diffuse reflectance FT-IR (DRIFT) spectra of powdered samples are frequently measured. A diffuse reflectance spectrum can be plotted using the Kubelka Munk (KM) or Beer—Lambert (BL) absorbance equations. Generally, the KM equation has been used to obtain linear relationships between band intensities and concentration for samples diluted with a non-absorbing matrix. In this paper the reproducibility of KM and BL absorbance spectral band intensities for undiluted samples is compared. The samples used are a thin film of polystyrene and long chain alcohols adsorbed on silica powder. The methods used to obtain reproducible, quantitative diffuse reflectance spectra and the use of calibration standards are discussed. It is found that the KM function depresses all weakly absorbing bands, resulting in the possibility of important spectral details being lost. The KM function can only generate positive absorbance bands and thus can differentiate between changes in concentrations of absorbing groups but cannot determine whether or not the concentration has increased or decreased on the sample relative to the background. KM spectra can incorrectly suggest that a sample is absorbing strongly in a spectral region when, in fact, the lack of a sample absorbance in this region, relative to the background, results in an apparent absorbance band. There does not appear to be any inherent advantage in using the KM rather than the BL absorbance presentation for the strongly absorbing samples used in this study. From the use of standards and pyrolysis experiments the adsorption density of stearyl alcohol on a powdered silica surface is calculated. It is found that FT-IR spectra of stearyl alcohol after reaction with the silica surface are different from the spectra of the same amount of alcohol simply mixed with silica. A comparison of the overall shape of the spectra and spectral band intensities, for long-chain alcohols reacted at 130°C with silica (the samples) and alcohols simply ground or dispersed from a solvent onto silica (the standards), with pyrolysis results shows considerable discrepancies. These may arise from the assumption that the molar extinction coefficient of absorbance bands for the samples is the same as for the standards. The best correlations between pyrolysis results and measured band heights of the standards were found if the BL spectral presentation was used.