The cone calorimeter and the fire propagation apparatus (FPA) are often used to carry out flammability studies of materials. There are various differences in the set-up of these two devices that could lead to different fire behaviour for the same material. Among these, the impact of the different heat sources used is studied here. The cone calorimeter employs an electrical cone heater and the FPA uses tungsten lamps to radiate a given heat flux level to the sample. Experiments are conducted in the FPA set-up using a conical heater or tungsten lamps as the heat source with clear PMMA and wood samples. Mass loss and temperature measurements are taken during the tests, and the bubble layer depth is measured after the tests. Significant differences in pyrolysis behaviour of both samples between the cone calorimeter and the FPA are consistently observed at the same heat flux level. These different pyrolysis behaviours can be explained by the wavelength dependency of the radiative material properties (reflectance, absorptance and transmittance). This conclusion is in agreement with, and provides an experimental confirmation, to theoretical findings in previous studies.