A methodology for quantifying calcium carbonate (875 and 712 cm(-1)) and silica (798 and 779 cm(-1)) by FTIR spectroscopy applying the constant ratio method is proposed. The studied method is applied for quantitative analysis of calcite and quartz in geological samples. The suggested method uses potassium ferricyanide (2115 cm(-1)) as standard and samples are prepared as potassium bromide pellets, with statistically satisfactory results (relative standard deviation less than 5%). The influence of particle size (pulverised samples) on the accuracy of the results found by FTIR spectroscopy applying the constant ratio method has been studied. The granulometric study suggests that the homogeneity of particle size is important. An absorbances correction method is proposed to correct spectral interferences. Due to the fact that most spectra are characterised by a high number of absorbance peaks, spectral interferences may occur so that some peaks cannot be used for quantitative analysis. These interferences can be evaluated and eliminated with the proposed method and the peaks can be used in the analysis.