The chlorine negative ion density and the electron density in chlorine- and boron trichloride-containing inductively coupled plasmas were investigated experimentally. Measurements were performed in a GEC reference cell operated in the inductively coupled mode. The chlorine negative ion density was measured as an excess electron density produced by photodetachment of electrons from the negative ions by 266 nm radiation from a frequency-quadrupled Nd:YAG laser. Both the excess electron density and the steady-state electron density were measured using a microwave interferometer. Various,aas mixtures were investigated including pure Cl-2, pure BCl3, combinations of the two, and combinations of these gases with N-2 and Ar. Cl- densities up to 4 x10(11) cm(-3) were measured depending on the gas mixture. The ratio of electron density to Cl- density was ac high as 3 in Cl-2 mixtures, and up to 5 in BCl3 mixtures. The plasma was probed at lower photon energy (355 nm) to photodetach electrons from other potential negative ions present in the plasma : to within the sensitivity of the measurement (1.3x10(8) cm(-3)), no other negative ions such as BClx- or Cl-2(-) ions were detected. Significant interactions between Cl-2 and Ar were observed as determined by changes in the Cl- density. The Cl- density also changed when BCl3 was added to Cl-2 discharges. Addition of Ar or N-2 to Cl-2 plasmas had little effect on Cl- density, but nitrogen addition had very dramatic effects when added to BCl3, causing a factor of 2 increase the Cl- density.