Lanthanum oxide thin film deposition by atomic layer epitaxy (ALE) was studied at 180-425 degreesC on soda-lime glass and Si(100) substrates using a beta -diketonate type precursor La(thd)(3) and ozone. The chemical constituents of the films were analyzed by TOF-ERDA, RES and FTIR while XRD and AFM were used to determine the crystallinity and surface morphology. Films grown below 275 degreesC were amorphous La2O2CO3, while at deposition temperatures above 300 degreesC XRD patterns indicated that cubic La2O3 phase was formed. All the films were transparent and uniform with only small thickness variations. Carbonate type impurity was found in all films, but the carbon content of the films decreased with growth temperature being 3 at.% in films grown above 400 degreesC. Hexagonal La2O3 was obtained when the films grown on silicon substrates were annealed at 800 degreesC or above in a nitrogen flow. The as-deposited cubic and annealed hexagonal La2O3 films were found to be chemically unstable in ambient air since a transformation to monoclinic LaO(OH) and hexagonal La(OH)(3) was detected, respectively.