Phosphorus-containing compounds are added to polymers as fire retardants and they are believed to act in both the gas and solid phases. In gaseous flame studies, they have been shown to be very effective gas-phase flame inhibitors; however, their performance varies with flame type. In particular, their effectiveness in co-flow diffusion flames is much lower than in other flames. To understand this behavior, co-flow diffusion flame experiments have been performed using dimethyl methylphosphonate (DMMP) added to the fuel stream. CO2 flame extinguishing tests show that phosphorus (via DMMP addition) is much more effective (similar to 4 times) than bromine (Br-2) at low concentrations. At higher concentrations (above similar to 5000 mu L/L), the efficiency ranking is reversed due to saturation of the DMMP inhibition effect, which is not observed in the case of Br-2 addition. The role of particle formation (via condensation of active phosphorus-containing species) in the loss of effectiveness of DMMP is investigated using Rayleigh scattering measurements. In order to understand the behavior of the flame stabilization region that is disrupted during flame extinguishment, premixed burning velocity simulations with detailed kinetics are performed for DMMP or Br-2 addition to the flames. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.