The authors report a positron annihilation investigation of quasicrystalline-to-crystalline phase transition in the Al-Mn-Si alloy system using both Doppler broadening and lifetime measurement techniques. The results show the presence of small vacancy clusters in the as-grown icosahedral phase. Isochronal annealing studies carried out from 100 degrees C onwards show that the vacancy clusters are annealed out along with the crystallisation process. The positron lifetime results rule out the possibility of a central hole which had been postulated in the icosahedral building block called the Mackay icosahedron.