Simultaneous pyrolysis and char combustion of < 500 μm coal and peat particles in an entrained-flow reactor was studied experimentally and theoretically at 700–1100 °C and at oxygen contents < 21 vol%. A method based on near infrared radiation was developed to measure the particle temperatures. Simultaneous pyrolysis and char oxidation occurs when oxygen reaches the char particle surface during the devolatilization stage. The overlap of homogeneous and heterogeneous combustion is favoured by low temperature and high oxygen content, as well as by a decrease in particle size. A simplified method of calculating simultaneous pyrolysis and char oxidation is presented. Model calculations show that under some conditions a maximum combustion rate may be achieved with a specific particle size, since the contribution of faster homogeneous combustion is increased relative to slower heterogeneous combustion.