Comparison of the room-temperature Mössbauer isomer shifts (I.S.) of the (\mathrm{Fe}^{2+}) and (\mathrm{Fe}^{3+}) daughter states of the ({ }^{57} \mathrm{Co})-microdoped monoxides (M O) with (M=\mathrm{Mg}, \mathrm{Ca}, \mathrm{Mn}, \mathrm{Co}, \mathrm{Ni}), or (\mathrm{Cu}) have been correlated with high-pressure data for (M=\mathrm{Co}) and (\alpha-\mathrm{Fe}{2} \mathrm{O}{3}). Any empirical relationship relating the I.S. to the mean iron valence (\langle m\rangle) relative to (10^{-8} \mathrm{sec}) that is applicable to iron oxides with a large iron-atom concentration needs to modified for microdoped samples where the (\mathrm{Co}-\mathrm{O}) bond length does not have its normal equilibrium value and the counter cations introduce an inductive effect. A nearly linear dependence of the I.S. on bond length is found for bond lengths near their normal value: (d() I.S. () /) (d(\mathrm{Co}-\mathrm{O}) \approx 1.08(\mathrm{~mm} / \mathrm{sec}) / \AA) for the ({ }^{57} \mathrm{Fe}^{2+}) daughter and (0.83(\mathrm{~mm} / \mathrm{sec}) / \AA) for the ({ }^{57} \mathrm{Fe}^{3+}) daughter. Covalent (\mathrm{Fe}-\mathrm{O} \sigma)-bonding is increased-and hence, the I.S. is decreased-either by bending of the (180^{\circ} \mathrm{Fe}-\mathrm{O}-\mathrm{Fe}) bond from (180^{\circ}) toward (90^{\circ}) or by introducing more electropositive counter cations (M) in the (180^{\circ} \mathrm{Fe}-\mathrm{O}-M) bonds. Application to ({ }^{57} \mathrm{Co}) microdoped copper oxides indicates that the (\mathrm{Co}^{3+2+}) redox couple tends to lie in the narrow energy range between the (\mathrm{Cu}^{2+/+}) and (\mathrm{Cu}^{3+2+}) redox couples. 1993 Academic Press. Inc.