Diesel oil as a fuel for compression ignition engines should meet the requirements laid down in the Regulation of the Minister of Economy of 9 October 2015 on the quality requirements for liquid fuels (Dz. U. of 2015, item 1680). Reports of UOKiK (the Office for Competition and Consumer Protection) reveal that a significant part of the diesel oil samples taken at filling stations do not meet the requirements for fuel resistance to oxidation. It is fuel manufacturers’ responsibility to ensure that the products sold by them are in conformity with the requirements laid down in the applicable normative documents; hence, factors adversely affecting fuel properties most likely occur in the fuel distribution chain. This may be caused by interactions between the fuel and the materials used for the construction of fuel storage tanks and handling equipment. Within the work described herein, the influence of copper on the oxidation stability of commercial diesel oil was examined.