A method to determine the concentration ratio of carbon monoxide to carbon dioxide in the gaseous product of a fluidized bed reactor for producing titanium tetrachloride. The hot fluidized bed of the reactor is used as the source of infrared radiation which radiation is directed through the gaseous products in upper portion of the reactor through a window in the reactor to an infrared spectrometer. The concentration ratio can be used to control the temperature of the fluidized bed reactor by controlling the amount of cool titanium tetrachloride that is introduced into the reactor.