The mass concentrations of various inhibitors required to extinguish explosions in stoichiometric methane-air and coal dust-air mixtures were measured in laboratory-scale systems of 8- and 20-L volumes. The additives studied were N 2 , CF 3 Br, CaCO 3 , KHCO 3 , and NH 4 H 2 PO 4 . The experimental method involved spherically-developing, adiabatic explosions at constant volume in which the test mixtures were taken to conditions of complete extinction. Strong, pyrotechnic ignition energies in the range of hundreds to thousands of joules were used in order to insure that inerting limits were truly independent of ignition energy. The order of effectiveness for the inhibitors was phosphate>nitrogen>CF 3 Br>carbonates. Essentially the same relative order of effectiveness was observed against methane as against coal dust; however, much higher absolute concentrations of the solid additives were required against methane than against coal dust. The phosphate, which was the most effective against either coal dust or methane, required 100 g/m 3 of the 7 μm powder to inert the most reactive coal dust concentration, and 280 g/m 3 of the same size powder was needed to extinguish stoichiometric methane. A preliminary theoretical analysis is presented based on a comparison of the measured inerting levels with those predicted from adiabatic, equilibrium thermodynamics using the established concept of limit flame temperatures. Three categories of thermodynamic systems are involved: fully-homogeneous ones, singly-heterogeneous ones, and doubly-heterogeneous ones. There are two types of singly-heterogeneous systems: those involving a solid fuel only and those involving a solid inhibitor only. The flammability and/or extinction behavior in the heterogeneous cases is controlled by limitations on the rates of devolatilization of the solid phase reactants. Preliminary data are presented for the rates of devolatilization of particles of coal dust, NH 4 H 2 PO 4 , and CaCO 3 pyrolyzed by a laser flux which simulates the flame heating flux. The data show that their measured rates of devolatilization are consistent with the inhibitor's measured effectiveness in extinguishing explosions.