A ‘‘test function’’ sensitive to the electron distribution function is used to determine the plasma parameters by Langmuir probes. It was successfully applied to determine the parameters of the hot-electron group in a multipolar, magnetically confined Ar plasma, even when the ratio of bulk to hot-electron densities is greater than 50. The method can also be used to measure the positive-ion density from a probe potential slightly below the plasma potential, and to estimate the electron and negative-ion density and temperature in an Ar/SF6 plasma. A characteristic plasma potential is also introduced and is correlated with the plasma parameters. © 1999 American Institute of Physics.