Abstract: A stack gas monitor system has an optimized gas flow portion which is permanently installed in a fossil fuel burning facility, and to which a spectrophotometric portion can be readily coupled for real-time determination in the system of a concentration of a first gas having a characteristic absorption spectrum and which is selectably introducible as a measured amount into a stack gas stream. A sample of the stack gas stream is flowing through the gas flow portion, with at least one gas of the sample stream having an absorption spectrum extending within an ultraviolet range of wavelengths of radiation and having absorbance values within this range which are higher than absorbance values of the first gas. The system can be purged and can be calibrated with the first gas. Stack gas blank data are obtained in the absence of the first gas. Composite gas data are provided upon introduction of the first gas into the stack gas.