CeO 2 has specific catalytic activity for the selective dehydration of 1,3-diols to unsaturated alcohols. In order to clarify the catalytic mechanism, quantum calculation based on the paired interacting orbital (PIO) theory was adopted to 1,3-butanediol–Ce 9O 18 cluster systems. Strong attractive interaction was observed between 1,3-butanediol and oxygen-defected CeO 2 (1 1 1), where three in-phase interactions were confirmed between H atom at 2-position in 1,3-butanediol and Ce cation and between two OH groups and other Ce cations. In addition, out-of-phase interaction between H and C atom at 2-position was induced by the in-phase interaction between H atom and Ce cation at appropriate coordinate of 1,3-butanediol. The PIO results suggest that the dehydration of 1,3-diols initiates from the abstraction of H atom at 2-position in 1,3-butanediol adsorbed on an oxygen defect site on CeO 2 (1 1 1) with tridentate coordination.