Positron lifetime and Doppler broadening measurements have been performed on alpha-irradiated tungsten to study the evolution of defects during isochronal annealing from 100°C to 1050°C. Vacancy-impurity complexes dissociate at the earlier stages and release the vacancies. Monovacancies start migrating at 400°C. The vacancy clusters coarsen to form microvoids in the region from 350°C to 650°C and remain stable upto 750°C. Some of the clusters collapse into vacancy loops. Dislocation/vacancy loops and microvoids start annealing around 750°C. The role of impurities and grain boundaries on the annealing behaviour of the defects is also discussed.