We report on in situ study of the initial transient during planar solidification of an Al-4 wt%Cu alloy by means of synchrotron X-ray radiography. Based on the recorded X-ray images, we first analysed in detail the time evolution of the macroscopic deformation of the solid-liquid interface. This deformation is due to the convective flow in the melt initiated by a residual transverse temperature gradient, and amplified by the solute rejection during solidification process. Additionally the growth rates were measured at three different positions along the deformed interface all along the solidification process. The experimental curves showed the increasing influence of convection as solidification proceeded. Longitudinal composition profiles in the melt were then determined during the solidification using a novel image analysis technique. The evolutions of solutal length, concentration at the interface and far away from the interface were then deduced. A comparison with Warren-Langer predictions revealed quantitative differences due to the convecto-diffusive transport in the melt. (C) 2010 Elsevier B.V. All rights reserved.