Minimum explosible concentrations (MEC) of dusts were measured in the Bureau of Mines 20-L chamber and in the Fike L-m3(1000-L) chamber. The MEC values for gilsonite dust and bituminous coal dust were measured in each chamber at several ignition energies. The explosibility of anthracite coal was also studied in the two chambers. Strong chemical ignitors with energies of 500 to 10 000 J were used in the tests. The uniformity of the dust dispersions in each of the chambers was studied by using optical dust probes. One purpose of the research was to determine if the 20-L chamber was “overdriven” at high ignition energies. The MEC-values measured in the 20-L chamber with 2500-J ignitors were comparable to those measured in the 1-m3chamber with 10 000 J ignitors. At higher ignition energies in the 20-L chamber, there was evidence of overdriving.