The net charge of porous Stober silica colloids is studied using a modified Poisson-Boltzmann theory in a spherical cell, with a focus on the case of water-filled porous silica particles suspended in a non-aqueous solvent. We show that the silica's usual negative surface charge, due to deprotonisation of the Si-OH group, is counteracted by an excess uptake of protons in the water-filled pores of the particle at low enough pH. A small volume fraction of pores suffices to induce a point of zero charge at pH approximate to 4. Based on the difference in Donnan potential between the porous medium and the solvent a relation can be constructed that describes the location of the point of zero charge analytically. The accuracy of this relation is confirmed by numerical calculations. For Stober silica in water we find a charge reversal below pH approximate to 3, which is in this case solely a result of the selective uptake of cations in the porous network. (c) 2012 Elsevier Inc. All rights reserved.