The correlation between chemical composition, layered structure evolution, and electrical properties of ultra-thin (2-5 nm) Lu oxide layers grown on chemically oxidized Si(100) and exposed to different thermal treatments was monitored by x-ray photoelectron spectroscopy, x-ray reflectivity and C-V, G-V measurements, respectively. These ultra-thin Lu2O3 films in contact with Si are not stable against silicate formation upon both ultra high vacuum (UHV) annealing and rapid thermal processing (RTP) in N2 atmosphere. A procedure to convert the Lu-silicate layer back to continuous Lu2O3 oxide on Si using high-temperature UHV annealing was identified.