The alteration and transformation behaviour of Ceca smectite in two acid and saline solutions (NaCl and KCl) was studied in batch experiments. This type of smectite is proposed as potential backfill material for nuclear waste storage sites. The initial solution was enriched with 500 ppm of U, Th and Eu. The evolution of pH and solution concentrations were measured over a period of 25 months. The mineralogical and chemical evolution of the clays was also studied. X-ray diffraction revealed a significant loss of diffraction intensity and the clays became amorphous to X-rays with increasing reaction time. Deconvolution of the XRD data indicated a continuous collapse of the smectite layers but no illitization. Infrared spectroscopy revealed unchanged smectite spectra at the end of the experiment. Thorium precipitated as amorphous ThO2, but showed an incipient crystallization after 500 days. The Th, Eu and U were neither adsorbed onto the clays nor incorporated into a secondary phase.