Triarylsulfonium salts Ar3S+MXn- with complex metal halide anions such as BF4-, AsFs(6)(-), PF6-, and SbF6- are a new class of highly efficient photoinitiators for cationic polymerization. In this article we describe several synthetic routes to the preparation of these compounds along with their physical and spectroscopic properties. Mechanistic studies have shown that when these compounds are irradiated at wavelengths of 190-365 nm carbon-sulfur bond cleavage occurs to form radical fragments. At the same time the strong Bronsted acid HMXn, which is the active initiator of cationic polymerization that takes place in subsequent "dark" steps, is also produced. A study of the parameters that affect the photolysis of triarylsulfonium salts is reported with a measurement of the absolute quantum yields. The cationic polymerizations of four typical monomers-styrene oxide, cyclohexene oxide, tetrahydrofuran, and 2-chloroethyl vinyl ether-with triarylsulfonium salt photoinitiators are described.