There is increasing interest in the use of hydrogen as an energy carrier. A hydrogen delivery system is required, and one solution is its addition to existing natural gas pipeline networks. A major concern is the explosion hazard may be increased should an accidental release occur, and this paper presents results from the mathematical modelling of confined, vented explosions of mixtures of methane with 0%2 20%2and 50%2hydrogen dilution by volume. The flow field in an explosion was predicted through solution of the averaged forms of the Navier-Stokes equations, with these equations closed using both k-epsilon and second-moment turbulence models. Accurate representation of the turbulent burning velocity of the various mixtures was necessary, and this was achieved using correlations obtained from the analysis of extensive experimental data sets on H(2)-CH(4) mixtures. Results, derived for explosions in a 70m(3) confined vessel with and without pipe congestion, demonstrate that hydrogen addition can have a significant effect on overpressure generation, particularly if turbulence generating obstacles are present.