Irradiated light water reactor fuel from the BR3 reactor was thermally annealed up to 2500 K in a Knudsen cell, and the effusing vapors were measured by mass spectrometry. The experiments provide data on the stoichiometry evolution of the fuel during release as well as a reliable method to evaluate the diffusion coefficients of volatile and less-volatile fission products.The analysis of the data starts from diffusion of xenon, which clearly shows three typical release stages respectively controlled by radiation damage annealing, self-diffusion, and matrix vaporization. The experimental measurements are also in agreement with the predictions of intragranular trapping models.Barium and cesium showed faster release than xenon, the former being likely to diffuse atomically to the grain boundaries where no evidence of formation of stable zirconates was found. These results were compared with those obtained by a burnup-simulated fuel, where barium was initially present in a perovskite phase, producing essentially different release patterns.