P-31 NMR spectroscopy was applied to the evaluation of water-tolerant Lewis acid catalysis of various metal triflates and chlorides in water using trimethylphosphine oxide (TMPO) as a probe molecule. Direct interaction of the TMPO molecule with the Lewis acid in water changes the original P-31 NMR chemical shift and line width. ScCl3, Sc(OTf)(3) and In(OTf)(3) exhibit larger changes in the P-31 chemical shift by the formation of a Lewis acid-TMPO complex than other triflates and chlorides. This is due to strong interaction between the Lewis acid and TMPO, which leads to the slow exchange between TMPO and H2O in ScCl3, Sc(OTf)(3) and In(OTf)(3). Lewis acids with large change in the P-31 chemical shift exhibit high catalytic activities for lactic acid formation from 1,3-dihydroxyacetone (1.3-DHA) and pyruvic aldehyde in water: Strong interaction with TMPO on the Lewis acid is a dominant factor for lactic acid formation. (C) 2013 Elsevier B.V. All rights reserved.