The microwave (MW) dielectric loss (or quality factor, Q) of BaCo1/3Nb2/3O3 (BCN) ceramics exhibits a strong dependence on sample-processing conditions and is not always consistent with the degree of 1:2 B-site cation ordering. Impedance spectroscopy results reveal that these BCN ceramics exhibit remarkably different electrical properties due to low levels of nonstoichiometry induced during ceramic processing. These compositional changes are difficult to detect by common chemical techniques but can have a significant influence on the electrical properties, and in particular on the extrinsic electrical conduction and Q. The influence of extrinsic conduction induced by ceramic processing should therefore be considered when optimizing Q in MW dielectric ceramics.