The paper presents the results of a complex analysis of gas content in 259 high-voltage transformers, in which overheating with temperatures above 700°C accompanied by discharges with different intensity, were detected. To increase the recognition reliability, ranges of gas percentage and gas ratio values have been determined and 15 nomograms corresponding to the high-temperature overheating, which were accompanied by discharges with different intensity, have been drawn. The analysis of the values of pair correlation coefficients between the concentrations of gases dissolved in transformer oil has been performed. The dynamics of nomogram changes during the development of combined defects with a high-temperature overheating has been analysed. The results of comparative analysis of the recognition reliability of hightemperatureoverheating, which are accompanied by discharges with different intensity, using the diagnostic criteria values regulated by current standards and methods of interpretation of the dissolved gas analysis results, are presented.