The limits of flammability of pulverized coals and other dusts were measured in a 7.8-liter laboratory-scale system that provided for accurate control of the significant experimental variables. Internal optical probes were initially used to develop an improved dust dispersion method. One probe was then used to routinely monitor the dust concentration prior to ignition. The problem of ignition source strength was resolved by independent assessments of the ignition energy requirements for realistic limit measurements for each dust. The carbonaceous dusts studied span the extremes from a completely volatilizable polyethylene powder with an H/C ratio of 2, to a low volatility anthracite with an H/C ratio of 0.3. Measured lean limits for the dusts in air vary inversely with their combustible volatile contents. Although their minimum explosive concentrations vary from 45 to 450 mg/l, their combustible volatile contents at the limits were nearly invariant at 40 to 60 mg/l. Also reported are data on the effect of particle size and varying oxygen concentrations. For Pittsburgh seam coal, the lean limit in air is approximately 130 mg/l, independent of particle size up to diameters of 40 μm. For coarser dusts, the limit concentration increases markedly with diameter. At enriched oxygen levels, the lean limits of the more volatile dusts, polyethylene and Gilsonite, were nearly insensitive to O2 content. For the coals however, there was a significant oxygen dependence at all O2 concentrations studied. The implications of these particle size and oxygen concentration dependences are discussed, particularly as they relate to the combustion mechanism and the coal devolatilization process. A series of calibration experiments were also performed with methane gas, paraformaldehyde, and various inhibiting powders. The resultant data, when combined with the data already described, are internally and externally consistent, reproducible, and reliable. Accordingly, it is recommended that the system described, or a somewhat larger version of it, be used as the future standard for dust flammability limit studies, including measurements of minimum dust concentrations, minimum oxygen concentrations, and inhibitor effectiveness.