Reflection high-energy electron diffraction (RHEED) and spot profile analysis low-energy electron diffraction (SPA-LEED) were used to study the order—disorder transitions of aluminium films on the Si (111) 7×7 surface, up to one monolayer (ML) coverage, deposited by molecular beam epitaxy. The phase transitions at the Al-covered surfaces were studied by analysing the temperature-dependent Bragg reflex intensities and reflex widths within the framework of the theory of phase transitions in two dimensions. Results are presented for Si(111)-√3×√3R30°-Al, Si(111)-√7×√7R19.1°-A1 and the Al/Si(111) “-γ phase”.