High‐resolution optical emission spectroscopy has been used to measure Doppler profiles of Ar and Ar+transitions in a divergent magnetic field electron cyclotron resonance (ECR) source, yielding average translational energies between 0.1 and 0.6 eV (Ar), and 1.0 and 2.5 eV (Ar+). ECR magnetic field configuration strongly affects Ar+energies, although little variation with Ar pressure (0.1–1.5 mTorr) is found. The average Ar energy increases slightly with pressure. These observations suggest that thermal ion and neutral excitation primarily results from collisional degradation of directed downstream ion energies arising from the unique ambipolar potential created in ECR sources.