5-Arylthianthrenium salts are a class of efficient triarylsulfonium salt photoinitiators for cationic polymerization. These compounds were prepared by a simple, straightforward, versatile, and high yield route. The new photoinitiators were characterized by standard analytical and spectroscopic techniques, and their activity as cationic photoinitiators was compared with related triarylsulfonium salts of similar structures using Fourier transform real-time infrared spectroscopy. Through the use of electron-transfer photosensitizers, the response of these photoinitiators can be readily spectrally broadened into the long-wavelength UV-visible regions of the spectrum. The results obtained suggest that 5-arylthianthrenium salts are potential replacements for now available triarysulfonium salt photoinitiators in many applications.