The un-stretched burning velocities and Markstein lengths of premixed CH4- and C3H8-air flames with added C3H2F3Br (2-BTP) or CF3Br (Halon 1301), have been studied experimentally and numerically. For CF3Br flame inhibition, the un-stretched burning velocities, predicted using a recently updated kinetic model for CF3Br flame inhibition, were in excellent agreement with the experimental results over a range of fuel-air equivalence ratio and CF3Br loading. For C3H2F3Br flame inhibition, the un-stretched burning velocities predicted using a recently developed kinetic mechanism were in good agreement with the experimental results for most of the equivalence ratios tested; nonetheless, for very lean flames approaching the flammability limit, model predictions differed by up to 25%, even for uninhibited flames. The influence of inhibitor on the flame response to stretch and susceptibility to instabilities was examined through consideration of the measured burned gas Markstein lengths. Markstein lengths were very large, leading to large stretch effects on the flame stability after ignition, and flame wrinkling during explosion tests, greatly increasing the rate of pressure rise. The influence of stretch with regard to flame inhibitor effectiveness is discussed. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.