The compound Pr_0.7Ca_0.3MnO₃ (PCMO) is known as a resitively switching material in thin film memristor architectures in which it changes reistivity upon the application of an external electric field  through the growth and rupture of localized filamentary type  perturbations that are related to the migration of oxygen vacancies. In this context PCMO operates as a reservoir of oxygen that can be used to tune redox-processes at the interface to the corresponding electrode. Although this mechanism has been widely accepted as a model concept, details on the lattice point disorder of the system (Pr, Ca)MnO₃ have not been reported in detail so far. This contribution presents under what thermodynamic circumstances oxygen vacancies form upon reduction and how their formation are related to the crystal chemistry of the system. The defect chemistry of solid solutions formed by the two orthorhombic perovskite-type compounds CaMnO₃ and PrMnO₃ is strongly determined by mixed valence states emerging from the presence of trivalent or tetravalent Mn-cations, Mn³⁺ and Mn⁴⁺. Both, thermogravimetric analysis as well as measurements of DC-conductivity at elevated temperatures in dependence of the partial pressure of oxygen quantitatively reveal the extent of oxygen vacancy formation in highly densified ceramic pellets originally consolidated by sintering in pure oxygen O₂. Iodometry additionally serves to analyse the average valence state of Mn-cations and thus of the deficiency in oxygen after targeted thermal treatments under specific redox conditions. The determination of electrical transport properties including also the careful inspection of the Seebeck-effect for such specimens demonstrates that the mobility of electrons is drastically reduced when the number of oxygen vacancies increases. For the specific composition Pr_0.7Ca_0.3MnO₃, an oxide potentially relevant for resistively switching memory device applications, this is the case when the deficiency in oxygen exceeds a concentration of 1000 ppm. Detailed crystallographic studies based on refined neutron-diffraction experiments for reduced ceramic material suggest, that the reduction in electron transfer rates between tri- and tetravalent cations of manganese originates from an anisotropy effect: In the case of comparatively large oxygen deficiency, vacancies preferentially form on lattice sites in the equatorial plane of MnO₆-octahedra rather than on their apices.