Tough market competition is caused by a number of requirements, the product quality being one of them. The content of dissolved gases in transformer oil is an indicator that can be used to assess the performance of instrument transformers and can serve as an important characteristic for estimating the transformer lifetime. At the beginning of this paper, a classification and basic characteristics of instrument transformers are given, with a special focus on the oil impregnated current transformer, type AGU, along with the related technological process, its phases and operations. A classification and characteristics of insulation materials for transformers are also given, with particular emphasis on the use of transformer oil. Chemical composition and characteristics of the oil contained in each instrument transformer are determined through the analysis of samples taken from the tested transformers. The reliability of test results depends on the applied method of analysis (in this case the gas chromatography analysis) and, for the most part, on the oil sampling from a hermetically sealed system, i.e. the oil impregnated instrument transformer. If the procedure is not consistently carried out according to standards, the obtained results can differ greatly from the actual characteristics of the transformer oil. A correlation between the occurrence of increased concentrations of hydrogen dissolved in the transformer oil and aluminium parts of the transformer has been found through a comparison with transformers in which this phenomenon was not registered. In addition to the classification of aluminium alloys, the paper discusses the conditions of metallurgical processing that could result in the occurrence of hydrogen in the aluminium alloys. Chemical reactions occurring on the aluminium surface (e.g. corrosion) and the chance of hydrogen being created during welding are also described. While planning the experiment, the intention was to determine between which technological phases the laboratory testing should be conducted. The experiment simulates physical and chemical effects of some technological processes applied during the various production phases of the transformer on its parts, particularly those made of aluminium alloys. The samples represent isolated units on which a test of specific materials as well as a test of technological processes can be conducted. In conclusion, the results of chromatography analysis of gases dissolved in a hermetically insulated system, i.e. the transformer oil, are used to test the hypothesis that these isolated units have a significant influence.