Extensive direct numerical simulations were performed to obtain the heat transfer coefficients (HTC) of bidisperse random arrays of spheres. We have calculated the HTC for a range of compositions and solids volume fractions for mixtures of spheres with a size ratio of 1:2. The Reynolds numbers are in the range of 30-100. It was found that the correlation of the monodisperse HTC can estimate the average HTC of bidisperse systems well if the Reynolds and Nusselt numbers are based on the Sauter mean diameter. We report the difference between the HTC for each particle type and the average HTC of the bed in the bidisperse system as function of solids volume fraction, Sauter mean diameter of the mixture, Reynolds number and investigate the heterogeneity of the individual particle HTCs. (C) 2016 Elsevier B.V. All rights reserved.