In this paper, an analytical formula for the effective thermal conductivity of porous media saturated with two immiscible fluids is derived for a wide range of pore sizes. The method applied to derive the formulas for the effective thermal conductivity is based on the mesoscopic adaptation of the hybrid approach by taking into account the influence of different types of scattering events on the interfaces between fluids and grain phase. The influence of surface roughness on the effective thermal conductivity is explored. Analysis of the results demonstrates a very good agreement of the theoretical approach with available experimental data. It is also observed that as the surface roughness increases, the heat carrier confinement at the grain surface increases and consequently the effective thermal conductivity decreases. (C) 2016 Elsevier Ltd. All rights reserved.