The behavior of electron-cyclotron resonant (ECR) and non-ECR H2 plasmas generated in a microwave plasma chemical-vapor deposition reactor have been studied as a function of magnetic field strength and absorbed microwave power. Plasma diagnostics were performed with a planar electrostatic probe, microwave power measurements, and visual inspection. The probe measurements were concerned with the plasma impingent on a sample table placed downstream from the active core of the plasma. ECR plasmas were found to be more efficient, stable, uniform, and dense than plasmas not generated by ECR. All properties of the ECR plasmas measured by the downstream probe showed a gentle monotonic increase with the magnetic field strength. This was attributed to a reduction in the spacing between the probe and the resonant zone. In the ECR plasmas the average electron temperature was 3 eV and the average density was 1016 m−3. The degree of ionization was on the order of 5×10−4.