The microstructural evolution of an Al-01 wt% Cu alloy was investigated during solidification at constant cooling rate by in situ synchrotron X-ray microtomography with a resolution of 2.8 mu m. Solidification of this alloy leads to a coarse dendritic microstructure which was fully characterized in terms of variation with temperature of the solid fraction, the specific surface area of the solid-liquid interface and the local curvatures of the solid phase. By analysing the evolution with solid fraction of individual dedrites, at least two coarsening mechanism were clearly identified in addition to solidification growth. The first mechanism involves remelting of small secondary dendrites arms to the benefit of bigger adjacent arms. The second is the coalscence secondary arms, with progressive filling of the inter-arm spacing and coalescence at the tips. Although this mechanism preferentially occurs at high solid fractions, these results show that the evolution of the dendritic microstructure during solidification is complex and involves the occurence of various mechanisms operating concurrently. In situ X-ray tomography thus allows revisiting the various models which have been proposed to account for dendrite coarsening during solidification. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rigths reserved.