  Radiation from the highly-charged ions contained in the plasma of
Electron-Cyclotron Resonance Ion Sources constitutes a very bright source of
X-rays. Because the ions have a relatively low kinetic energy ($\approx 1$ eV)
transitions can be very narrow, containing only small Doppler broadening. We
describe preliminary accurate measurements of two and three-electron ions with
Z=16--18. We show how these measurement can test sensitively many-body
relativistic calculations or can be used as X-ray standards for precise
measurements of X-ray transitions in exotic atoms.
