The influence of experimental parameters on the shape of reflection-absorption infrared (RAIR) spectra of thin organic films on metallic substrates is analyzed in detail. Two main sources for artifacts in RAIR. spectroscopy are identified : reproducibility in the angle of incidence and variations in the substrate refractive index. At high angles of incidence, optical artifacts are more prominent. The divergence of the probe beam is of importance if quantitative analysis of intensity contour is intended. In addition, surface cleaning can alter the optical properties of the substrate which will cause spectral artifacts. These difficulties are greatly diminished if the spectral information of the thin film is retrieved from p- and s-polarized spectra.