High and low resolution Fourier transform infrared spectroscopy have been used to monitor the gaseous pyrolysis products of fumaroyl dichloride. High resolution spectra of the transient species tricarbon monoxide and the stable species chloroacetylene and chlorodiacetylene have been recorded and some of the bands analyzed using an interactive Loomis-Wood spectral line fitting program, MacLoomis. The attributes of the program MacLoomis are described and, for C3O, the ν1 band and a number of hot bands associated with ν1 have been analyzed. Low resolution infrared spectra show that dichloroacetylene and dichlorodiacetylene are also minor products of high temperature pyrolysis while 2-propynoyl chloride is the major product at low pyrolysis temperatures. The low resolution spectra of chloroacetylene and chlorodiacetylene have been reanalyzed in the light of the improved data. Pyrolysis of 2-propynoyl chloride also results in the production of C3O. A reaction pathway for the production of tricarbon monoxide is discussed.