An experimental study of binary mixture solidification in a V-shaped sump is conducted. NH4Cl-H2O is used as the phase change material. The variations of temperature, concentration, as well as the location of the interface front, are measured and reported in this investigation. The results indicate that : the solidification process; exhibits totally different behavior when the initial component of the solution is. varied. For a hypoeutectic solution, the columnar solidification in the dendritic interface is the dominant mechanism. For the hypereutectic solution, both columnar and equiaxed solidification are important. The solidification starts as the equiaxed dendrites grow and coalesce in the entire solution. They then descend and settle at the bottom of the sump to form a loose, mushy zone. The solid region grows underneath the mushy zone.