In this work we have studied the effect of grain boundary space charge on oxide ion conductivity of La2-delta ErxMo2O9 (0.05 <= x <= 0.25) ionic conductors. Microstructure is studied by scanning electron microscopy and high resolution transmission electron microscopy. Impedance spectra are analyzed in the frame work of impedance spectroscopy genetic programming method. A distribution function of relaxation times model is obtained, which is comprised of two peaks and correlated with the contribution from grain and grain boundary conductivities. The activation energies obtained for both grain and grain boundary conductivities confirm thermally activated hopping of oxygen. Theoretical analysis is carried out to estimate the grain boundary space charge potential at different temperatures for all compositions. The depletion of oxygen vacancies in the space charge layer is also determined. Despite the fact that Er is an isovalent dopant, both the space charge potential and oxygen vacancies are found to be composition dependent. The increase in space charge potential is related to decrease in grain boundary conductivity.