This study is the first to examine the inhibition effectiveness of C2HF3Cl2 (HFC-123) on premixed hydrocarbon-air flames and is motivated by the eventual phase-out of CF3Br (Halon 1301) used in civilian aircraft cargo compartments. To study the inhibition effectiveness, we measured the laminar burning velocity of CH4-air flames with added C2HF3Cl2 in a spherical, constant-volume combustion vessel, over a range of inhibitor concentration and fuel-air equivalence ratio. Burning velocities at ambient (T = 298 K; P = 1.01 bar) and elevated (T = 400 K; P = 3 bar) conditions were compared to numerical predictions obtained using a newly-developed kinetic mechanism describing the decomposition of hydrochlorofluorocarbons (HCFCs) in hydrocarbon-air systems. The agreement was very good, considering the model parameters were not adjusted, and the present study was the first to test the mechanism against experimental data of a two-carbon HCFC. In addition to providing model validation, the effectiveness of C2HF3Cl2 was compared to the analogous HFC compound C2HF5 to explore the advantages of Cl substitution for F. Experimental measurements of agent influence on burning velocity, as well as numerical modeling of premixed flame structures, demonstrated that C2F3Cl2H is a more effective flame inhibitor than C2F5H, particularly for very lean CH4-air mixtures. The reaction pathways and sensitivities were analyzed to interpret the differences in the inhibition mechanisms of C2F5H and C2HF3Cl2 and to prioritize elementary reactions for further study. (C) Published by Elsevier Inc. on behalf of The Combustion Institute.