Rate constants and their temperature dependencies were measured for the chemiluminescent reaction of ozone with the flame and furnace effluents of two commercially available sulfur detectors, the Sulfur Chemiluminescence detector (registered trademark of Sievers Instruments, Inc.) and the Chemiluminescent Sulfur Detector (registered trademark of Antek Instruments, Inc.). The absolute rate constants and activation energies agreed within experimental error with the well-established values for the SO + O(3) reaction in all detector configurations. When ozone was present in the reaction zone, the reaction of the furnace effluent with NO(2) also was in good agreement with the rate constant for the SO + NO(2) reaction. However, when ozone was not present in the reaction zone, little or no reaction occurred with NO(2). These results imply that an unknown sulfur species, X, is formed in the combustion zone and/or transfer line of the instrument and converted to SO upon reaction with ozone in the chemiluminescence detection cell. Based on our experimental results and known reaction rate constants, we can eliminate S(2), S(2)O, SO(2), S(2)O(2), HS, HSO, HSO(2), HOSO(2), and H(2)S as X. We hypothesize that X is S(3) formed in the association reaction of S atoms with S(2) within the transfer line.