Experiments using varied particle size, heating rate and pressure with a high-volatile bituminous coal were undertaken to determine possible effects of these experimental parameters on coal pyrolysis experiments. The results show that secondary reactions can influence the results to a large extent. For the particle size in particular it is established that the optimum is not a bed of very fine particles, since detailed gas analysis by FT-i.r. spectroscopy reveals a clear trend in the composition of the product gas which can be related to secondary tar cracking in the bed. Furthermore, it is concluded that vapour pressure equations cannot be used directly to describe the evolution of tar.