The aim of this paper is to report the results of experiments carried out on Al-Cu alloys with different Cu contents,studying the microstructure evolution during holding in the semi-solid state.The 3-D microstructure was observed by in situ X-ray microtomography carried out at ESRF Grenoble,France.The variation of the solid-liquid interface area per unit volume during holding was determined.In addition,local observations show that two coarsening mechanisms of the solid particles occur simultaneously:dissolution of small particles to the benefit of larger ones by an Ostwald-type mechanism and the growth of necks between solid particles due to coalescence.These observations confirm that in situ X-ray tomography is a very powerful tool to study the microstructure evolution in the semi-solid state and the influencing mechanisms in real-time. The aim of this paper is to report the results of experiments carried out on Al-Cu alloys with different Cu contents, studying the microstructure evolution during holding in the semi-solid state. The 3-D microstructure was observed by in situ X-ray microtomography carried out at ESRF Grenoble, France. The variation of the solid-liquid interface area per unit volume during holding was was determined. In addition, local observations show that two coarsening mechanisms of the solid particles occur simultaneously: dissolution of small particles to the benefit of larger ones by an Ostwald-type mechanism and the growth of necks between solid particles due to coalescence. These observations confirm that in situ X-ray tomography is a very powerful tool to study the microstructure evolution in the semi-solid state and the influencing mechanisms in real-time.