Abstract The solid-state reaction of the second kind in a sandwich type diffusion couple of Co 1−z Ga 2z 3 O and β-Ga 2O 3 has been investigated between 1249 and 1550°C in air. The quantity z, which corresponds to the saturation concentration of β-Ga 2O 3 in CoO, was determined as a function of temperature by X-ray methods and the optical microscope; the homogeneity range of the spinel phase Co 1−y Ga 2+ 2y 3 O 4 was investigated also. The growth of the thickness of the reaction layer follows a parabolic rate law; the activation energy is 71.6 kcal/mole. A comparison of reaction rate constants of the first and second kind in connection with experimental results, achieved with a modified marker technique, leads to confirmation of the Wagner mechanism for the formation of CoGa 2O 4 spinel as supposed before by Laqua. Reaction rate constants of the second kind, calculated from interdiffusion profiles in CoO-β-Ga 2O 3 diffusion couples, are in good agreement with experimental values. Presented data are used for estimating interdiffusion coefficients for the CoO-β-Ga 2O 3 system according to theoretical aspects developed by Pelton, Schmalzried, and Greskovich.