The mechanism of the enantioselectivity of lipase has been investigated by MALDI TOF-MS with photoaffinity probes. When 1-phenylethyl m-benzoylbenzoate was used as a photoaffinity probe, both the enantiomer that reacts as a substrate of lipase and the non-reactive enantiomer combined with lipase. However, in the case of isobornyl m-benzoylbenzoate, only the reactive photoaffinity probe combined with lipase.