Abstract: In order to investigate the influence of gaseous impurities in natural gas-hydrogen mixtures on the hydrogen embrittlement susceptibility of pipeline steels. The mechanical properties of L245 pipeline steel under air and 4 MPa hydrogen environments were analyzed using slow tensile testing on notched round bar specimens in the present work. Furthermore, the changes in hydrogen embrittlement susceptibility of L245 pipeline steel were studied when exposed to 4 MPa hydrogen environment with various gaseous impurities such as H₂O, O₂, CO₂, etc., and the fracture surfaces were systematically analyzed using Scanning Electron Microscopy. The results were as follows: 1) Compared to air environment, L245 pipeline steel exhibited reduced plasticity under 4 MPa H₂ environment, with a fracture mode characterized by a combination of quasi-cleavage fracture at the edge of the notch and ductile fracture at the center of sample, indicating certain degree of hydrogen embrittlement susceptibility; 2) The effect of H₂O varied depending on its concentration. Addition of H₂O at or below 100 ppm had minimal impact on the hydrogen embrittlement susceptibility of L245 pipeline steel. However, when H₂O concentration increased to 1000 ppm, the sensitivity to hydrogen embrittlement significantly increased; 3) Under an environment consisting of 4 MPa H₂+100 ppm H₂O, addition of O₂ at a concentration level of 100 ppm resulted in significant reduction in hydrogen embrittlement susceptibility for L245 steel. While addition of CO₂ at a concentration level also at around 100 ppm led to considerable increase in sensitivity to hydrogen embrittlement for L245 steel.