Thin film mixtures of cobalt(II) + nickel(LT) oxyhydroxide have been prepared by electrodeposition from water on to platinum. The chemical diffusion coefficient (D) over bar of the hydroxide ion through the mixed oxide has a marked temperature dependence; and Eyring-type graphs have two linear regions. The temperature at which there is a change from one linear region to the other corresponds to the Neel transition temperature T-N for cobalt-nickel oxide. It is seen that the chemical diffusion coefficient (D) over bar decreases with increasing temperature at some compositions rich in NiO, and this is discussed in terms of an increased mobility of oxide (or maybe Ni2+) species competing with the movement of OH- in an anionic type of ’mixed alkali’ effect. It is suggested that systems evincing both a Neel transition (at T-N) and a Verwey transition (at T-V) ought to show T-N approximate to T-V.