PE-HD water pipes are exposed to adverse environmental conditions, both during production and their subsequent use in the civil engineering. Several organic compounds migrating into water may lead to deterioration of water quality, especially its taste and odour. Commonly used antioxidant BHT and its products of degradation are leached into water as a result of degradation of the PE-HD pipes material. This paper presents the results of laboratory tests of BHT concentration changes in water circulating in the PE-HD close loop pipe of 30 m length as well as of numerical studies based on data obtained by the laboratory measurements. Numerical calculations were performed using the commercial software Fluent, Ansys Inc., based on the computational fluid dynamics (CFD) principles. The analysis of water samples confirmed the migration of the antioxidant from the pipe material into the water. The results of numerical analysis gave good agreement with the measured values.