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Trace rare gas optical emission spectroscopy has been used to obtain electron temperatures (T-e) and percent dissociation of Cl-2 in chlorine-containing, high-density inductively (transformer) coupled plasmas. In this method, a small amount of an equal mixture of the rare gases is added to the plasma and emission spectra that include lines from the rare,oases are recorded. Modeling of the dependence of the rare gas emission intensities on T-e allows T-e to be derived from the best match between the observed and computed intensities. T-e in Cl-2 plasmas increases from 1.7 eV at 20 mTorr to 3.4 eV at 0.5 mTorr. These values are 1.3 to 1.8 times lower than those recorded with a Langmuir probe. This discrepancy is due at least in part to artifacts associated with using a single Langmuir probe in a reactor with semiinsulating walls. Absolute percent dissociations of Cl-2 were determined by monitoring the intensity of an emission band of Cl-2, normalized to emission from Ar and Xe. Cl-2 percent dissociation increased with power and decreased with pressure. At 1.0 mTorr, Cl-2 is about 85% dissociated at a power density of 0.1 W/cm(3) and 95% dissociated at 0.3 W/cm(3) in a Cl-2 plasma. Addition of BCl3 to Cl-2 increases the dissociation of Cl-2.