Abstract: Organometallic compounds such as tricarbonyl-(.eta..sup.4 -1,3-cyclo-hexadiene)lron(O) display an infrared spectrum which varies according to the solvent polarity, pH and similar factors. By comparing the wavenumber position and shape of the peak of a test spectrum with reference spectra by interpolation or extrapolation, information is obtained about the test sample, even a quantitative analysis in some cases. More than one organometallic compound may be used in one experiment (for double checking), and/or a mixture of analytes may be investigated, for obtaining information about the combination.