The hydrothermal reactions of acetaldehyde using cubic CeO2 nanoparticles were performed on the assumption that repolymerisation of monolignin by aldehydes can be suppressed by catalytic oxidation in the presence of CeO2 during depolymerisation of lignin. The catalytic potential of cubic CeO2 nanoparticles was evaluated by comparing them with octahedral CeO2 via their structural, morphological, catalytic characterisation and in terms of catalytic reactivity. In the cubic CeO2 system, the conversion of acetaldehyde and the selectivity of acetic acid (oxidation of acetaldehyde) were higher than those in octahedral CeO2, while the selectivity of crotonaldehyde produced by aldol condensation was lower than that in octahedral CeO2. The overall reaction mechanism was also developed based on a kinetic study on the primary products of acetaldehyde using the first rank Delplot method. According to its catalytic oxidation rate constant, cubic CeO2 has 3-5 times higher oxidative catalytic potential than octahedral CeO2, suggesting, that the repolymerisation of monolignin by aldehydes can be effectively suppressed by cubic CeO2 nanocatalysts.