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la metallurgia italiana 23 High-temperature XRD investigations on phase transformations R. Montanari The first part of this paper briefly describes how X-ray diffraction (XRD) is used for the determination of phase diagrams and gives some examples of particular interest. The attention is focused on in situ XRD experiments, i.e. performed directly at the high temperatures where phase transformations take place. The related experimental problems are presented and discussed. Furthermore, high temperature XRD offers the possibility to investigate structural aspects of phase transformations involving both solid and liquid. In order to illustrate the precursor phenomena occurring in melting and solidification and the correlations between the structures of solid and liquid phases, the second part of the paper presents some results obtained from investigations on pure indium (99.99 wt%) and In90-Sn10 alloy. Parole chiave: high temperature X-ray diffraction, phase diagrams, phase transformation, melting, solidificatio