Atomic chlorine concentrations in Cl2 and CF3Cl plasmas have been measured using both infrared absorption spectroscopy and optical emission actinometry. These measurements were made over a range of plasma conditions including plasma excitation frequencies of 72 kHz–13.5 MHz, power inputs of 10–100 W, and pressures of 200–800 mTorr. In Cl2 plasmas, the technique of optical emission actinometry misrepresents atomic chlorine concentration changes by nearly an order of magnitude. The errors in the actinometry technique are believed to result from excited state Cl production by electron impact dissociation of Cl2. A simple model for Cl emission is in good agreement with the experimental observations. In CF3Cl discharges, the technique of optical emission actinometry is shown to accurately represent variations in atomic chlorine concentration with changing process conditions.