In all measured NAM-IR spectra of PS/PVP mixtures, spectral elements from both PS and PVP are visible, with those for PVP becoming dominant as higher concentrations of this polymer are added. For instance, the large peak at 1685 cm−1, caused by carbon–oxygen double bonds and deriving from PVP, increases in intensity, while the sharp PS-derived peak at 1484 cm−1, stemming from carbon–hydrogen bonds in the aromatic ring, is fading out.