Phase equilibria of the zinc oxidecobalt oxide system were studied by a combination of X-ray diffraction and in situ electrical conductivity and thermopower measurements of bulk ceramic specimens up to 1000 degrees C in air. Rietveld refinement of X-ray diffraction patterns demonstrated increasing solubility of Co in ZnO with increasing temperature, which is supported by the slight increase in wurtzite (Zn1xCoxO) cell volume and lattice parameter a versus temperature determined for the phase boundary compositions. Similarly, the solubility of Zn in CoO increased with increasing temperature. In contrast, the spinel phase (ZnzCo3zO4) exhibited retrograde solubility for Zn. Electrical measurements showed that the eutectoid temperature for transformation of rocksalt Co1yZnyO into wurtzite and spinel is 894 +/- 3 degrees C, and the upper temperature limit of the stability of the spinel phase is 894 degrees C898 degrees C for the compositions Co/(Zn+Co)=0.821. The resulting composition-temperature phase diagram is presented and compared to the earlier (1955) diagram by Robin.