Aim of this study was to characterize the changes occurring in tissues undergoing microwave ablation, in order to more accurately predict the actual treated volume. Different experiments were conducted in restricted samples (10-40 mm side cubes) and in large samples of ex-vivo bovine liver, varying the deposited energy (60 W at 2.45 GHz, applied for 1-10 min). A comparison between pre- and post-ablation dimensions was conducted, pointing out a substantial shrinkage, non-uniform over time and space, asymmetrical along the microwave antenna axis and perpendicularly to it. The obtained results show a complex tissue behavior and a correlation between the carbonized area and the contraction phenomenon. © 2015 EurAAP.