This article presents measurements of absolute Cl2 and Cl number densities in a chlorine transformer-coupled plasma. It is part of a series of reports on measurements of densities and energy distributions of all charged and neutral species in the same plasma system over an extensive range of pressure and power. Cl2 and Cl number densities were determined from optical emission spectroscopy and advanced actinometry. Number densities relative to the Xe actinometry gas are reported as a function of pressure (1–20 mTorr) and power (10–1000 W) during slow etching of SiO2-covered Si wafers. A detailed treatment of the effects of gas temperature on the conversion of these ratios into absolute number densities is also included. Cl2 is largely (∼90%) dissociated at the highest powers, with a somewhat higher degree of dissociation at low pressure. The Cl number density becomes nearly independent of power at high powers (especially at lower pressure) due to the combination of a higher degree of dissociation of Cl2 and an overall drop in number density due to heating of the gas. A zero-dimensional (global) model is used to compute Cl2 and Cl number densities. It gives a Cl wall recombination coefficient of 0.04 on the plasma-seasoned stainless steel walls. © 2000 American Institute of Physics.