It is suggested that measurements of thermal magnetoresistance can be used to investigate valence changes in paramagnetic ions which are strongly coupled to the lattice, and a technique is described which has been found suitable for making these measurements. The technique has been used to study valence changes in impurity ions in magnesium oxide produced by x irradiation and the results lead to the tentative conclusion that one of the ions present is chromous (Cr²⁺).