The growth of thin Ti Γlms on Al(001), Al(110) and Al(111) surfaces at room temperature has been studied using high-energy Rutherford backscattering spectroscopy (RBS) and channeling, x-ray photoelectron spectroscopy (XPS) and low-energy electron di β raction (LEED). Our results show that Ti atoms form a thin, metastable, fcc overlayer on Al(110) and Al(001) surfaces. The primary evidence for this conclusion is the reduced backscattering that occurs as the Ti atoms shadow the Al atoms in the fcc structure of the Al substrate. For the Al(111) surface the Al surface peak area in ion channeling, measured as a function of Ti coverage, shows a small decrease for the Γrst monolayer (ML) of Ti coverage but then increases gradually with coverage, a characteristic of alloy formation. However, XPS and low-energy ion scattering (LEIS) results are generally consistent with overlayer growth of Ti on the Al(111) surface, and the LEED pattern indicates an ordered overlayer for Ti coverages from 2 to 12 ML, at which point the Al surface was completely covered by Ti. The results suggest the growth of incommensurate, ordered islands of hcp Ti on Al(111) in a Stranski-Krastanov growth mode, in remarkable contrast to the pseudomorphic fcc Ti overlayer growth observed for Ti Γlms on Al(001) and Al(110).