Plasma potentials are measured in a 2.45 GHz electron cyclotron resonance (ECR) multicharged ion source. Both axial and radial potential profiles are determined for an argon plasma by using a Langmuir probe at various pressures. Multicharged ions up toAr12+are observed in the extracted beam. The plasma potentials are positive with respect to the plasma chamber wall and are on the order of several tens of volts. The values in profile measurements of the plasma potential are consistent with the spatial averaged values obtained by ion beam measurement. Along the mirror axis the plasma potential, the electron temperature, and density increase as the ECR zone is approached, and are roughly flat or hollow near the ECR zone. The radial profiles are loosely peaked or flat at the position off the ECR zone, and asymmetrical or hollow near the ECR zone. Due to some serious problems for the probe measurement, the plasma potential is deduced by several methods and the density is estimated from ion saturation currents. ©1998 American Institute of Physics.