Attenuated total reflection (ATR) Fourier transform infrared (FT-IR) in situ measurements were performed during the catalytic hydrogenation of acetophenone under high pressure (5.0 MPa). The catalyst used was a suspension of rhodium nanoparticles in an ionic liquid. At the highest temperature used (80 degrees C), the selectivity of the hydrogenation to 1-phenylethanol dropped from 80% in the beginning of the reaction, when acetylcyclohexane was the main side product, to less than 50% after a few hours of experiment because of the consecutive hydrogenation of 1-phenylethanol to ethylcyclohexane. The evolution of the concentrations of reactant and products was quantified using flawless spectra of pure components with a classical least squares (CLS) multivariate method applied to several ranges of the mid-infrared spectra. The only variable parameters of the analysis are the concentrations of each component themselves and the baseline shift of the spectrum during the reaction. The advantage of using multivariate analysis over the analysis of a single vibrational band, as well as the limitations of this type of spectral analysis, are discussed.