Hf- and Ta-aluminates have been grown by 3keV O2+ reactive ion beam mixing (IBM) of X/Al interfaces (X=Hf or Ta). The kinetics of growth, composition and electronic structure of the films formed have been studied using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and factor analysis. A reactive IBM kinetics of two stages has been found by means of factor analysis. In a first stage, HfO2 or a mixture of Ta suboxides and Ta2O5 species are formed for the Hf/Al or Ta/Al interface, respectively. Ta suboxide species are subsequently transformed into Ta2O5 species at the beginning of the second stage. In a second stage, HfO2 or Ta2O5 species are transformed progressively into Hf–O–Al or Ta–O–Al species, respectively, leading to the synthesis of custom designed Hf- and Ta-aluminates. The evolution of the Auger parameters and UPS valence band spectra shows that this transformation is accompanied by changes in the electronic structure of the oxide films formed.