Abstract: The effects of polydispersity on radiative and interfacial convective heat transfer are investigated in particle–gas two-phase media for solar particle receiver applications. Non-grey radiative transfer is analysed using the collision-based Monte Carlo ray-tracing method. The Mie theory is employed to calculate radiative properties of particles. The finite volume method and the explicit Euler time integration scheme are used to solve the transient energy equations for the particle and gas phases. Three alternative approaches to modelling particle properties and thermal conditions are employed: (i) a novel discrete size model, in which particle groups within discrete size intervals are assigned individual properties and temperatures locally; (ii) a lumped size model, in which integral properties and a single temperature are assigned to the particle phase locally; and (iii) a monodisperse size model, in which properties are evaluated for the Sauter mean diameter of the polydispersion and a single temperature is assigned to the particle phase locally. Strongly size-dependent radiation absorption and interfacial convective heat transfer are predicted with the discrete size model for alumina particles. Particles smaller than 27.4μm located near the aperture absorb the solar irradiation and transfer heat to the gas phase most effectively. The angular spread of the incident solar radiation is found to have a negligible effect on the overall absorption, although the most uniform thermal conditions occur for the solar irradiation with the smallest confinement angle. The overall absorptance of alumina particles is higher by 3.4% and 2.7% than that of iron (III) oxide and mullite particles, respectively. The lumped and monodisperse size models allow for reduction of the computational time at the expense of lower accuracy and limited information about particle properties and thermal conditions.