Effects of positive flame stretch on the laminar burning velocities of hydrocarbon/air mixtures were studied experimentally using outwardly propagating spherical flames. The test conditions included propane, methane, ethane, and ethylene-air flames at various fuel-equivalence ratios and normal temperature and pressure. Karlovitz numbers generally were less than 0.3 so that the flames were remote from quenching conditions. Within this range, the ratio of the 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 reactant mixture. In addition, Markstein numbers varied in a roughly linear manner with fuel-equivalence ratios over the range of the measurements, which were somewhat removed from flammability limits where behavior might differ. Effects of stretch were substantial: Markstein numbers varied from -2.5 to 7.2, yielding corresponding laminar burning velocity variations of 0.4-2.7 times the value for an unstretched (plane) flame over the test range. The ranges of fuel-equivalence ratios for unstable preferential-diffusion conditions (negative Markstein numbers) were as follows: propane, greater than 1.44; methane, less than 0.74; ethane, greater than 1.68; and ethylene, greater than 1.95. Fuel-equivalence ratios for maximum flame temperatures and laminar burning velocities are near unity for the present flames; therefore, neutral preferential-diffusion conditions are shifted toward fuel-equivalence ratios on the unstable side of unity, in qualitative agreement with recent approximate theories treating the effects of stretch on laminar premixed flames.