Abstract The differential calorimetric technique has been applied to measure the dissolution enthalpy, Δ H δ → α irrad , of zirconium hydrides precipitated in structural components removed from the Argentine Atucha 1 PHWR nuclear power plant after 10.3 EFPY. An average value of Δ H δ → α irrad = 5 kJ/mol H was obtained after the first calorimetric run. That value is seven times lower than the value of Δ H δ→α = 37.7 kJ/mol H recently determined in Zircaloy-4 specimens taken from similar unirradiated structural components using the same calorimetric technique [P. Vizcaı́no, PhD thesis, Number TD-11/03, Inst. Tech. ‘Prof. Jorge A. Sábato’, CICAC, CNEA, Buenos Aires, Argentina, 24 June 2003]. Post-irradiation thermal treatments gradually increase that low value towards the unirradiated value with increasing annealing temperature similar to that observed for TSSd irrad [P. Vizcaı́no, A.D. Banchik, J.P. Abriata. J. Nucl. Mater. 304/2–3 (2002) 96–106]. Therefore the same H atom trapping mechanism during reactor operation already proposed to explain the evolution of TSSd irrad is also valid for Q δ → α irrad . As the ratio Q/Δ H is proportional to the number N H of H atoms precipitated as hydrides, the increment of Q δ → α irrad with the thermal treatment indicates that the value of N H also grows with the annealing reaching the value corresponding to the bulk H concentration when Δ H δ → α irrad ≈ 37 kJ/mol H. That is a direct indication that the post-irradiation thermal treatment releases the H atoms from their traps increasing the number of H atoms available to precipitate at the end of each calorimetric run and/or isothermal treatment.