High-resolution optical emission experiments have been performed to investigate the application of this technique to the characterization of electron cyclotron resonance (ECR) sources. Preliminary experiments have used Ar and 02 discharges operating between 0.5 and 1.5 mTorr, with 700–2300 W of microwave power, and two distinct magnetic field configurations. Results of these measurements indicate that ECR discharges are optically thick for transitions which terminate on long-lived (metastable) energy levels, producing significant self-absorption. Measurements of emission intensity for Ar and Ar + lines which are not self-absorbed show that selected transitions can be used to reliably monitor the relative concentrations of these species. © 1989, American Vacuum Society. All rights reserved.