The positron annihilation technique (PAT) was used to study the annealing behaviour of defects prodeuced by 30.0 MeV alpha-irradiation in molybdenum. The small increase in the Doppler broadening lineshape parameter, S, in the temperature region 150–300°C is assigned to the migration of vacancies and the formation of vacancy clusters. In the temperature region 300–750°C S showed a prominent decrease. this later recovery can be seen in two steps, from 300°C to 550°C and from 650°C to 750°C, indicating two different types of defects were present. The first step can be assigned to the agglomeration of vacancy clusters whereas the second step may be due to the thermal coarsening of microvoids or due to He bubble growth.