The report evaluates and ranks commercially available sulfur dioxide monitoring instrumentation for fixed combustor sources. It presents a logical review of available instruments and specifications in terms of established use-criteria, selection and procurement of the most promising instruments and finally a field test program where the instruments were installed and operated continuously under real, fixed combustor stack conditions. Instrument performance was rated on criteria including accuracy, specificity, reliability, maintainability, ruggedness and other selected performance criteria. The ranking of instruments was necessarily dependent on the order of importance the potential user places on the established performance criteria. (Author Modified Abstract)