In order to optimize the synthesis process of internal-Sn Nb3Sn wires, magnetization measurements have been performed on samples reacted at various temperatures between 650 and 725 °C and for time ranging from 8 to 200 h We compared samples of various origins, from NIN in Xi'an and Alstom in Belfort. These measurements give us access to critical and irreversibility temperatures. Although all Nb-Sn phases are superconductive, their critical temperatures are varying from 6 K (18at%Sn) to 18.3 K (25%atSn : stoichiometric Nb3Sn). Thus, Tc and T* measurements are related to the progress of tin diffusion and reaction in the wires. We correlate the results of magnetization with SEM (Scanning Electron Microscopy) and EDS (Energy Dispersive Spectroscopy) observations, which indicate directly the Sn content distribution across Nb filaments.