A new cell developed for in situ FTIR investigations was used to investigate the oxidative decomposition of propylene carbonate with different amounts of water. Further subtractively normalized interfacial (SNIFTIRS) spectra were obtained for the oxidation products of 1 M LiPF6 in ethylene carbonate (EC) and of 1 M LiPF6 in dimethylcarbonate (DMC). Finally, the decomposition products of a solution of 1 M LiPF6 in a 1:1 EC–DMC mixture were investigated and the corresponding spectra assigned to decomposition products of the single-solvent electrolyte solutions.