The main objective of the presented work is to evaluate the effect of hydrogen service conditions on the mechanical properties of API steel grades used for well completions. The evaluation methodology implies a preconditioning of the steel specimens to long-term exposures under high-pressure hydrogen atmospheres and compare the results of subsequent mechanical tests with those of steels not being exposed to this gas. The aim of this research is to compare the performance of different API grades when subjected to hydrogen service. The outcomings of the study will help to evaluate long-term integrity of completion systems and materials compatibility for hydrogen storage applications. Mechanical tests like notched-tensile tests, hardness determination and impact tests were performed, in order to detect the embrittlement of the metals by comparing results between specimens not previously charged with hydrogen and specimens being subjected to a hydrogen atmosphere under high-pressure. The notch tensile specimens were pre-stressed to 80% of the nominal yield strength, in order to force and assure the hydrogen diffusion into the notch area where localized increased tensile stresses are concentrated. Furthermore, by means of carrier gas hot extraction analysis the hydrogen content in the samples was measured, to give an indication of the absorption capacity of these grades under the stated conditions. The API grades L80, P110 and Q125 have been selected to represent a wide and popular selection of ductility and yield strength. All samples were subjected to a series of mechanical tests to determine the presence of hydrogen embrittlement. The results show different behavior of the materials after being exposed to a hydrogen atmosphere, from the noticeable decrease to even a "no effect" on the mechanical properties. The results of notch tensile tests of the steels L80 and Q125 are showing some level of hydrogen embrittlement, compared to P110, being the one least affected by the presence of this gas. The measurement of hydrogen content in the samples delivers similar results for all the grades. Microscopic analysis shows the structure of the crystal lattice of the steels studied, helping to understand, together with the state of stress, how sensitive the material is to be affected by hydrogen embrittlement. There is no literature that describes the hydrogen effect on the mechanical properties of API steels used for tubings and casings in well completions, nor their sensitivity to hydrogen embrittlement. The results of this research are of great importance to give an idea of the compatibility of the steels that can be used for high-pressure hydrogen operations, such as hydrogen underground storage as well as to evaluate the potential recompletion or use of existing wells.