High-purity aluminium (99.9999%) was irradiated with 600 MeV protons at the Swiss Institute for Nuclear Research (SIN) with a damage rate of 3,5 x 10^-6 dpa/s. Irradiation with 600 MeV protons produces helium, hydrogen, and other impurities through mutational reactions. The irradiation experiments were carried out at 120 degree C (0,42*Tm where Tm is the melting temperature in K). Transmission electron microscopy on specimens irradiated to 0.2 and 0.6 dpa has shown the presence of (a) cavity-denuded zones (CDZ) along grain boundaries, (b) cavity-containing zones (CCZ) adjacent to the CDZ and (c) regions beyond the CCZ containing a very low density of cavities. At the dose level of 2 dpa, a dense population of very small cavities is resolved on the grain boundaries and also in their immediate vicinity (in the CDZ). Furthermore, at the dose levels of 0.6 and 2 dpa, a well defined dual size distribution of cavities is observed in the CCZ. The results are discussed in terms of agglomeration of helium atoms which are considered, during irradiation, to diffuse mainly via vacancies.