Laminar burning velocities and the flame response to stretch, as characterized by Markstein numbers, were determined experimentally and computationally for outwardly propagating spherical laminar premixed flames. The mixtures studied were premixed hydrogen/air/diluent and hydrogen/30% oxygen and 70% nitrogen (by volume)/diluent flames, with the latter condition of interest for pre-external vehicular activity preparation activities on board manned spacecraft. Other flame conditions were room temperature (300 K), fuel-equivalence ratios of 1.0 and 1.8, pressures of 0.5, 0.7, and 1.0 atm, diluents including helium, argon, nitrogen, and carbon dioxide as suppression agents, and diluent concentrations of 0-40% (by volume), which implies oxygen indices of 30-10 for present conditions. Predicted flame behavior was obtained from one-dimensional, spherically symmetric, steady, and time-dependent numerical simulations with variable-property and multicomponent transport and with detailed hydrogen/oxygen chemical kinetics. Flames Studied were sensitive to stretch, yielding unstretched/stretched laminar burning velocity ratios of 0.6-1.25 for conditions well away from quenching conditions (e.g., Karlovitz numbers; Ka <= 0.5). Diluents became more effective (provided greater reductions of the laminar burning velocity for a given diluent concentration) in the order helium, argon, nitrogen, and carbon dioxide, which reflects their increased capabilities either to quench the reaction zone by increased specific heats or to reduce flame velocities by reduced transport rates. The addition of diluents generally decreased Markstein numbers, which made the flames more susceptible to preferential-diffusion instability. This effect increases flame speeds and tends to counteract the effect of diluents to reduce laminar burning velocities. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.