A convenient method is reported for the measurement of gas-phase Cl atom concentrations in the 10(11)-10(13) atoms cm(-3) range in a flow reactor. The titration reaction with vinyl bromide is used to measure the absolute CI atom concentration, and the relative Cl atom concentration is monitored by the HCl(v) infrared emission intensity produced by the reaction of Cl atoms with hydrogen sulfide. A microwave discharge through dilute flows of Cl-2, CCl4, CFCl3, and CF2Cl2 in Ar were characterized as sources of Cl atoms in the flow reactor. The elementary rate constants for reaction of Cl atoms with HN3, CF3I, and C2F5I were measured as (1.1 +/- 0.3) x 10(-12), (5.1 +/- 1.5) x 10(-13), and (3.9 +/- 0.8) x 10(-12) cm(3) molecule(-1) s(-1), respectively, at 300 K. These rate constants were obtained after adjustment of the apparent rate constants for secondary reactions.