The chemical diffusion coefficient (D) over tilde of polycrystalline nickel oxide was studied as a function of oxygen activity (10(-5) atm < p(O2) < 1 atm) and temperature (700 degreesC < T < 1200 degreesC) employing electronic conductivity relaxation experiments in the van der Pauw configuration. With this technique the sample is equilibrated under a given oxygen pressure. The oxygen content of the atmosphere surrounding the sample tablet is then changed stepwise and re-equilibration of the sample is followed by monitoring the conductivity relaxation. The chemical diffusion coefficient is obtained from the time-dependent variation of the electronic conductivity. The temperature dependence of (D) over tilde between 700 and 1200 degreesC can be described by (D) over tilde (cm(2) s(-1)) = 2.40 x 10(-3) exp(-0.70+/-0.05 eV/kT). The chemical diffusion coefficient is found to be almost independent of the oxygen partial pressure and thus of oxide composition. The electronic conductivity at 1000 degreesC shows a 1/6 dependence on the oxygen partial pressure, indicating the presence of doubly ionized nickel vacancies and electron holes as predominant defects. The (D) over tilde values are compared with literature results on single crystals. (C) 2000 Elsevier Science B.V. All rights reserved.