This paper describes a series of experiments performed on BM05 at the European Synchrotron Radiation Facility (ESRF), dedicated to the analysis of a mushy zone evolution in a fixed temperature gradient. A mushy zone is the partially solid/partially liquid zone that is formed when solidification proceeds with the development of dendrites, and it has been recently shown that synchrotron X-ray radiography is a powerful technique, perfectly adapted for such a study. In situ and real-time characterisation clearly evidences the microstructural changes of the mushy zone during the holding stage, and measurements of the mushy zone boundary positions using image processing enable us to analyse the successive regimes. Each regime is directly related to solute diffusion mechanisms, namely temperature gradient zone melting, solute diffusion in the inter-dendritic liquid channels due to the solid-fraction gradient and solute diffusion in the melt.