The rotating cone reactor is a novel reactor type for rapid thermal processing of solids. This paper focuses on the experimental determination of the gas-to-particle heat transfer coefficient. This quantity has been measured for several particle diameters (average size of 150, 280 and 425 mu m) and cone rotational frequencies (11.3 and 28.3 Hz). The gas-to-particle heat transfer coefficient obtained from these experiments varied between 280 and 1030 W m(-2) K-1. Experimental parameters which were kept constant in this study were the particle mass flow rate (5 g s(-1)), the cone geometry (cone top angle pi/3 radians) and the temperature of the particle feed (280 degrees C). The experimentally determined heat transfer coefficients were represented in dimensionless form as a function of the particle Reynolds number. The particle Reynolds number has been obtained from measurements of the local gas-phase velocity and the local particle velocity. Analysis of the experimental results revealed that the gas-to-particle heat transfer coefficients could roughly be represented by the well-established Ranz-Marshall correlation (Rant and Marshall, 1952, Chem. Engng Progress 48, 173) for isolated non-rotating particles. The difference between the experimentally observed particle Nusselt numbers and the theoretically predicted Nusselt numbers based on the Ranz-Marshall equation is probably due to the influence of particle rotation on the gas-to-particle heat transfer process. Calculations showed that the time in which the particles lose half of their rotation frequency is typically in the order of the particle residence time in the rotating cone reactor.