paper describes results of the study on polybutylene terephthalate and polybutylene terephthalatepolyethylene having high chemical resistance. The article shows the dynamics of changes in values (Ap), (Ey), (σt) (σp), (εp) depending on the composition and exposure time under unloading which is close to natural conditions in aggressive liquid media. A decrease in the strain of destruction of PBT + HDPE compositions depending on the exposure time was identified. The Sharpie method identified a monotonous decrease in Ap polymer composite material with an increasing content of HDPE and best compatible compositions. The article describes a significant increase in the composite material matrix and addition of non-polar polyethylene into the matrix of polybutylene terephthalate which improves the resistance to aggressive media.