Effects of positive flame stretch on the laminar burning velocities of H2/O2/N2 flames at normal temperatures and various pressures and nitrogen dilutions were studied both experimentally and computationally. Measurements and numerical simulations considered freely (outwardly)-propagating spherical laminar premixed flames at both stable and unstable preferential-diffusion conditions with fuel-equivalence ratios in the range 0.45-4.00, pressures in the range 0.35-4.00 atm, volumetric oxygen concentrations in the nonfuel gas in the range 0.125-0.210, and Karlovitz numbers in the range 0.0-0.6. For these conditions, both measured and predicted ratios of unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding Markstein numbers that were independent of Karlovitz numbers for a particular pressure and reactant mixture. Measured Markstein numbers were in the range -4 to 6, implying strong flame/stretch interactions. For hydrogen/air flames, the neutral preferential-diffusion condition shifted toward fuel-rich conditions with increasing pressure. Predictions of stretch-corrected laminar burning velocities and Markstein numbers, using typical contemporary chemical reaction mechanisms, were in reasonably good agreement with the measurements.