The behavior of Ga on the Si(001) 2×1 surface has been studied for Ga coverage of 0.8 monolayers and annealing temperatures in the range 623–863 K by means of scanning tunneling microscopy. Annealing at the lowest annealing temperature Ta leads to the formation of the Ga 2×2 phase, which is a result of self-arrangement of Ga dimers, as well as irregular Ga clusters. An increase in Ta to 673 K results in the formation of a partially ordered metastable Ga 5×2 phase, whereas further increase in Ta leads to the transformation of the 5×2 structure into 8×n (n=4,5) structure, the degree of order of which gradually increases with an increase in the annealing temperature. At Ta=833 K the surface is uniformly covered by the periodically aligned arrays of 8×5 units, this 8×5 structure is believed to consist of the ordered double-layered Ga clusters having 24–26 Ga atoms in the first layer and four Ga atoms in the second layer. The 8×5 units are out of registry with the underlying Si surface in one direction, this uncoupling from the surface is believed to be caused by Ga–Ga interactions. Further increase in Ta up to 863 K results in the evaporation of the Ga atoms from the surface giving rise to the redimerization of the Si and Ga atoms and reappearance of the Ga 2×2 phase. © 1999 American Vacuum Society.