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Features of the influence of hydrogen degradation of pipe steel on brittle strength of circumferential field welded joints were considered within the framework of analysis of the possibility of using the Ukrainian gas-transportation system for transportation of mixtures of natural gas and hydrogen. Applied for these purposes were the methods of finite-element modeling of the structure stress-strain state during welding and further service together with modern criteria of macroscopic fracture of a cracked body. Results of prediction of brittle strength margin of a typical welded section of the main gas pipeline with postulated surface cracks at transportation of gas-hydrogen mixtures of different composition showed that the areas of the weld and heat-affected zone are the most prone to brittle fracture. However, as regards fatigue strength of welded joints, greater ranges of stress intensity factors in the heat-affected zone under the impact of cyclic loading by inner pressure or bending moment, result in an essential reduction of the residual strength margins at prediction of long-term brittle strength. 15 Ref., 1 Tabl., 7 Fig. [ABSTRACT FROM AUTHOR]