The impedance of both single-crystalline and polycrystalline manganese-zinc ferrite samples has been measured as a function of oxygen non-stoichiometry and the impedance of the bulk and the grain boundary has been analysed as a function of oxygen nonstoichiometry. The impedance of the single crystal increased in order along (1 0 0), (1 1 0) and (1 1 1) planes. Both the bulk and the grain boundary resistance increased with the oxygen non-stoichiometry, and the impedance for a higher impedance sample decreased at lower frequency due to the contribution of the capacitive component of the grain-boundary. The grain-boundary resistance increased more sharply with the non-stoichiometry than the bulk resistance due to the higher sensitivity to oxidation in the grain boundary. The grain boundary capacitance was about three orders of magnitude larger than the bulk capacitance and the both were almost constant against the non-stoichiometry.