Structural variations in 39-MeV alpha-particle irradiated (Tirr= 60°C) polycrystalline molybdenum during post-irradiation annealing were studied by X-ray and TEM methods. Despite the high density of irradiation induced defects in the structure of the specimen X-ray measurements showed zero relative lattice parameter change after an irradiation dose of 1.1 × 10−2dpa. However, during the annealing Δa/awas changed in the positive range, exhibiting two peaks—at 100 and 300°C—whereas the damage structure detected by TEM indicated no changes. Analysis of the results leads to the conclusion that in the range 100 to 250°C migration of isolated vacancies and their annihilation at interstitial clusters as well as possible formation of new vacancy clusters occur. The second peak on the Δa/atemperature dependence curve is related to the transformation (probably, thermal disintegration) of vacancy clusters formed at energetic displacement cascades during irradiation.