Abstract: The results are presented for the researchers research concerning the quality control of transformer mineral oils by various methods of optical spectroscopy. The main spectral ranges are presented which contain useful information for analysis as well as some information about the measurement features and analysis principles. It was shown that according to the studies of optical spectra one can determine a number of standardized indicators of oil quality, assess the degree of their hydrocarbon base degradation as well as to diagnose the presence of defects in a transformer.