Laboratory-scale experiments were conducted to examine the influences of ignition energy, coal dust concentration and particle size of rock dust on suppression of coal dust explosions in a Siwek 20 L spherical explosion chamber. The amount of rock dust required to inert an explosion increased as the ignition energy used to initiate the explosion was increased. Excessively high amounts of rock dust were required when using high ignition energies. This is due to a phenomenon known as overdriving the explosion. Comparison of these results with those of other researchers showed that 5 and 2.5 kJ ignition sources give inerting levels similar to those obtained in mine-scale experiments and in the US Bureau of Mines 20 L chamber, respectively. Results showed that tests with an ignition source > 5 kJ may overestimate the amount of rock dust required to inert an explosion. The amount of rock dust required decreased as coal dust concentration increased beyond that required to produce a stoichiometric volatiles-air mixture. The experimental results also showed that as its particle size decreases, less rock dust is required to inert an explosion. Pocahontas and Pittsburgh coal dusts had similar inerting levels even though Pocahontas has a lower volatile matter than Pittsburgh. This can be attributed to experimental conditions in the 20 L Siwek chamber, which drive off more volatile matter from the coal than in the standard proximate analysis test.