The combination of O16(±,±)16O oxygen resonance measurement and transmission electron microscopy (TEM) provides an effective method to study the kinetics of nucleation and growth of the Cu2O phase during reduction of CuO thin films. The oxygen-loss kinetics in the CuO thin films upon vacuum annealing has been monitored with use of the oxygen resonance at 3.05 MeV. From the oxygen-loss measurement, the volume fraction of transformation of CuO to Cu2O as a function of time and temperature has been determined. In situ TEM observation measures the growth of Cu2O grains. When these results were analyzed with the model of phase transformation of Johnson, Mehl, and Avrami, the activation enthalpy of nucleation of the Cu2O phase in the CuO matrix has been deduced as En=2.3 eV/atom. The specific interfacial energy between the CuO and Cu2O phases has been estimated to be in the range 0.30.85 eV/atom, and the number of atoms in the critical nucleus of Cu2O has been calculated to be about 23 atoms. © 1992 The American Physical Society.