The mechanism of the gas concentration dependent conductivity of semiconducting metal oxides is often described by one-dimensional analytic models. The thermopower of materials with space charge regions is mostly not taken into account. Here, a numerical method is presented that allows to calculate the effective thermopower and effective resistance of semiconducting gas sensitive oxides with space charge regions. This involves both the numerical solution of the nonlinear Poisson-Boltzmann equation leading to a potential distribution and two potential equations for the calculation of the electrical conductivity and the thermovoltage. Varied morphological parameters were the grain sizes and the neck radii. The results were plotted in the form of Jonker diagrams and compared with homogeneous metal oxides. Intrinsic materials and slightly donor-doped materials turned out to be ideal candidates for direct thermoelectric gas sensors. First tests with Fe2O3 as an intrinsic material confirmed this assumption. (C) 2008 Elsevier B.V. All rights reserved.