High-temperature water (HTW), which can be loosely defined to include both liquid water above its ambient-pressure boiling point, and supercritical water, has been gaining recognition as an environmentally friendly reaction medium. At the same time, research of water-tolerant Lewis acids (WTLAs) has been steadily broadening the applications for these catalysts. Our research combines HTW with catalysis by WTLAs to achieve organic transformations in high yields. Thus far, our work has focused on alkyne hydration to form ketone, aryl-methyl ether deprotection, and Friedel-Crafts alkylation by diphenylmethanol. Alkyne hydration forms the Markovnikov product in excellent yields at 200°C with 0.5mol% In(OTf)₃ loading. Deprotection, a pharmaceutically important transformation, is studied with the model compound anisole which transforms to phenol at 250°C in the presence of 5 mol% In(OTf)₃. Diphenylmethanol successfully alkylates 1-naphthol and phenol at 200°C with 5 mol% In(OTf)₃. Our work focused on comparing catalysis by In(OTf)3 to that by other WTLAs, such as Sc(OTf)₃, Yb(OTf)₃, and InCl₃, as well as on revealing the kinetics and thermodynamics of these systems and comparing them with the conventional analog.