Mechanisms of island formation and growth are central issues in thin-film deposition. The shape and size as well as the spatial distribution of stable islands in the first phases of growth determine a wide variety of film properties. In an earlier work, the growth mode of sputter-deposited tin (Sn) and lead (Pb) films on polycrystalline Al substrates was analyzed in the framework of the rate-equation theory. This analysis yielded Stranski-Krastanov growth with the size of the critical nucleus in the range from 6 to 20 monomers. The present study is devoted to a detailed experimental investigation of the mechanisms involved in the formation of Sn and Pb islands on polycrystalline aluminum (Al). All thin-film systems treated in this work were deposited on glass substrates by DC magnetron sputtering. They consist of a polycrystalline Al underlayer covered with a 100 Angstrom thick Sn or Pb films. The variation of the mean Sn and Pb island distance was determined as a function of the deposition temperature and of the underlayer thickness. It varied from 0.5 to 20 mu m and showed an Arrhenius dependence on the deposition temperature. A rate-equation analysis yielded activation energies for surface diffusion of single Sn and Pb atoms of approximately 0.5 eV. Island size distributions (ISDs) were determined for different deposition temperatures and were mostly found to be monomodal with a global maximum at the mean island size. All these facts indicate that the mechanism of island formation is mediated by surface diffusion of the Sn or Pb monomers on the Al surface in a wide range of deposition temperatures.