The unsteady, conjugate, forced convection heat/mass transfer from ensembles of mono-size Newtonian fluid spheres to Newtonian fluids has been studied at moderate Peclet and Reynolds numbers. The cell model has been used to account for the spheres hydrodynamic interactions. The momentum and heat/mass balance equations were solved numerically in spherical coordinates system by a finite difference method. The values considered for the sphere exterior Reynolds number are Re-2 <= 200. The influence of the voidage and physical properties ratios on the heat/mass transfer rate was analysed for sphere Peclet numbers, Pe(1(2)) <= 10(4), and three fluid - fluid systems: gas-liquid, liquid-liquid and liquid-gas. (C) 2010 Elsevier Ltd. All rights reserved.