AbstractDissociative adsorption and desorption of H2O and H2 on Li2O were studied using the CRYSTAL92, ab initio quantum chemical calculation code. The activation energy for desorption of H2O by recombination reaction between two surface –OH was estimated to be 154 kJ mol−1. This value showed good agreement with the reported activation energy for tritium desorption measured by TPD experiments. It was also shown that the activation energy for H2 adsorption was much larger than that for H2O adsorption. This fact indicates that H2 adsorption requires a higher temperature than D2O adsorption.