A reaction mechanism is reported that describes BCl3/CI2/Ar plasmas used in the etching of metal lines in microelectronics fabrication processes. Although many of the fundamental electron-impact cross sections for this system are not well known, a reasonable set of reaction paths and rate coefficients has been derived to describe low-pressure reactors with high plasma density. The reaction mechanism describes 59 possible gas-phase events and 18 plasma-surface interactions. A well-mixed reactor model is used to develop the reaction set and to test it against absolute experimental measurements of electron and Cl- densities, as well as relative measurements of BCl and Cl radicals in an inductively coupled research reactor. The experimental data cover a wide range of operating conditions and gas mixtures. The model provides quantitative agreement with measurements over the whole range of conditions and diagnostics, capturing most of the observed trends. in addition, the model predicts relative ion ratios and Cl/Cl-2 density ratios as measured by molecular beam mass spectrometry. Comparisons of results from the zero-dimensional model and a two-dimensional continuum plasma model, using the same reaction mechanisms, further validate the chemistry set and show the strengths and weaknesses of the well-stirred reactor approach. Sensitivity analysis shows the dominant reactions contributing to model predictions of species densities.