This paper describes a simple experimental technique to determine the ignition temperature of various cylinder-shaped beds (diameter 17-160 mm, height 10-80 mm) of a South African coal and it reports the application of thermal ignition models in interpreting and extrapolating the small scale results. Measurements of the rate heat generation in isothermal conditions are also reported and compared with the results of ignition tests. The coal used in this work was found to undergo spontaneous ignition in small scale tests at temperature from 120 to 220°C. The thermal ignition model provides a good correlation for critical ignition temperature of bodies of different sizes. The correlation has proved to be relatively insensitive to complications in reaction (oxygen interparticle diffusion) and to physical changes in the material (reactant consumption), but it depends on the boundary conditions (Biot number). An effect of coal particle sizes indicates the existence of intraparticle diffusion limitations. Comparison between ignition and thermal data yields a satisfactory agreement in prediction of critical dimension by menas of the thermal ignition model. Small scale tests have been extrapolated and examples of prediction of critical dimensions at different temperatures are also reported. © 1997 Combustion Institute.