Fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) were considered both experimentally and computationally for laminar premixed flames. Mixtures of hydrogen and oxygen with nitrogen, argon and helium as diluents were considered to modify flame transport properties for computationally tractable reactant mixtures. Freely (outwardly)-propagating spherical laminar premixed flames were considered for fuel-equivalence ratios of 0.6 to 4.5, pressures of 0.3 to 3.0 atm, volumetric oxygen concentrations in the nonfuel gases of 0.21 to 0.36, and Karlovitz numbers of 0 to 0.5, at normal temperatures. For these conditions, both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), yielding constant Markstein numbers for particular reactant conditions. The present flames were very sensitive to flame stretch, exhibiting ratios of unstretched-to-stretched laminar burning velocities in the range 0.6 to 3.0 for levels of flame stretch well below quenching conditions. At fuel-lean conditions, increasing flame temperatures (by dilution with argon rather than nitrogen) tended to reduce flame sensitivity to stretch whereas increasing pressures tended to increase tendencies toward preferential-diffusion instability behavior. At low pressures, helium-diluted flames had reduced tendencies toward preferential-diffusion instability behavior compared to nitrogen- and argon-diluted flames due to stabilization of flame properties by strong effects of preferential diffusion of heat. Predicted and measured flame properties exhibited encouraging agreement using contemporary reaction mechanisms. Finally, flame structure predictions suggest that H and OH radical production and transport are important aspects of preferential-diffusion/stretch interactions, reflecting the strong correlation between laminar burning velocities and H+OH radical concentrations for present test conditions. (C) 2001 by The Combustion Institute.