Electrochemical investigations on polycrystalline orthorhombic FePO4 (heterosite), the lithium-poor part of the LiFePO4/FePO4 redox couple, gives insight into its charge-carrier chemistry. The material obtained by chemical delithiation exhibits a predominant electronic conductivity. A residual lithium content of 0.03 wt% was found and has to be considered as lithium interstitials in the FePO4 ground structure. Compensation by electrons induces n-type conduction, confirmed by the pO2 dependence of the electronic conductivity. The pO2 dependence is primarily ascribed to the formation of an oxidic surface composition leading to bulk depletion of lithium, rather than to filling of oxygen vacancies.