Fluoroptic temperature measurement has been applied to determine the external heat transfer coefficient of particles flowing along the surface of a rotating cone reactor, specified by a half cone top angle of 45 degrees and a maximum diameter of 68 cm, which has been designed for flash pyrolysis of biomass. Two different hydrodynamic regimes have been considered. Both, the cooling of a very dilute stream of hot particles, flawing freely along the cold cone wall and the cooling of hot particles in a very dense cold sand flow (moving bed regime) were studied. Tn the very dilute regime (without sand supply), the derived heat transfer coefficients are in the range of 500-1000 W m(-2) K-1 and display a minimum as a function of the cone rotation frequency. Experiments at cone rotation frequencies of 3.77-5.28 Hz show that heat transfer coefficients for small particles (average particle diameters of 159 and 284 mu m) are reasonably well predicted by the correlation of Rant and Marshall [W.E. Ranz, W.R. Marshall, Evaporation from drops: Part 2, Chem. Eng. Pr. 48 (1952) 173] for heat transfer by gas phase convection to a non-spinning sphere in free flight. Contrary, larger particles with an average diameter of 428 Ccm show significantly higher heat transfer coefficients than expected on basis of the Rant and Marshall equation. This is explained by a changing flow pattern of the particles over the conical surface and the consequences for the slip velocity between gas phase and particles. Large deviations from the Ranz-Marshall equation at a cone rotation frequency of 3.01 Hz are explained in terms of an increased contact with the wall resulting in a higher contribution of conduction to the total heat transport. For sample parties in a flow of sand with an average diameter of 350 mu m, the determined heat transfer coefficient gradually decreases as a function of the cane rotation frequency; it remains constant however for coarse sand (750 mu m). These phenomena have been explained in terms of variation in density of the gas/solids emulsion.