Diffusion reaction between Zircaloy-2 and thoria ThO₂) has been studied by employing miniature diffusion couples under uniaxial loading conditions. Electron probe microanalyser (EPMA) has been used to establish the concentration profiles. Microstructural characterisations by scanning electron microscope (SEM) and concentration profiles by EPMA indicate the absence of any intermediate phase in the diffusion zone. The experiments show the preferential grain boundary diffusion of Th and O in Zircaloy-2 matrix. The interdiffusion coefficients are strongly dependent on the temperature of annealing and are of the order of 10^-18m^2/s. The activation energy values for the layer growth kinetics and those estimated for interdiffusion coefficients are comparable.