The effect of substrate bias on plasma parameters has been studied for an electron cyclotron resonance plasma under typical materials processing conditions. Substrate conditions include floating with respect to the plasma, negative dc bias, or capacitively coupled rf bias. It has been found that the dc‐bias can profoundly affect the electron density, the electron temperature, and the plasma potential, well into the bulk of the plasma. The rf bias is found to be generally less perturbative, though can still cause significant change in the plasma potential. Changing the rf bias frequency appears to alter the effects on the bulk plasma.