Isochronal annealing studies on alpha-irradiated nickel specimens have been performed via the positron annihilation technique using both the Doppler broadening and life-time measurement methods. A two-state trapping model favours the presence of dislocation loops and vacancy clusters like microvoids in the irradiated specimens. The decrease of the positron parameters between 400 and 650 K has been associated with He trapping by vacancy-type defects and He-vacancy complex formation. Above 650 K stabilization of this complex has been observed. The results agree with the model where the He atoms, though initially at interstitial sites, quickly interact with vacancy-type defects and relax to substitutional positions. Present observations favour the idea that a He atom in a metal vacancy (substitutional) acts as a trapping site for positrons.