The Rietveld method has been applied to X-ray spectra in order to study the precipitated mass fraction of delta' and delta in Al-Li alloys. The method allows us to obtain quantitatively the delta' and delta precipitate mass fractions, and their evolution with aging time. Furthermore, this method also gives directly the cell parameter evolution of the matrix phase and indirectly the mean half radius of S' precipitates through an appropriate calibration curve. Experimentally, this calibration has been approached by previously studying the evolution of the mean half radius of delta' by transmission electron microscopy (TEM). Thermoelectric power has also been shown to be a powerful technique to study the microstructural evolution of Al-Li alloys, bring sensitive to the different stages of precipitation associated to the delta' and delta phases. The comparison of the different experimental results allow us to stablish a clear difference between the precipitation kinetics and the hardening kinetics. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd All rights reserved.