A surprisingly efficient carbonylation of ethylene to yield propionic anhydride and propionic acid using halide promoted Mo(CO)(6) catalysts has been recently reported in the literature. Although the earlier report included detailed kinetics and a unique mechanistic interpretation involving a metalloradical process, the spectroscopic examination, which played an important role in clarifying the reaction mechanism, was only described qualitatively. This report describes the in situ infrared spectroscopic investigation of the chemistry of the molybdenum carbonyl catalysts in greater detail. Emphasis will be on the spectroscopic examinations of the equilibria of Mo(CO)(6) and the molybdenum halocarbonyls, the oxidation of Mo(0) with alkyl halides, and in situ regeneration of the active catalyst under reaction conditions. As part of this investigation, a general method for the determination of equilibrium constants for reactions involving infrared active species using attenuated total reflectance technique, ATR, is described wherein neither the extinction coefficient nor the effective pathlength are known with certainty. The method is demonstrated using the equilibrium between Mo(CO)(6) and Mo(CO)(5)I- as an example.