Recent computer modeling of high-density chlorine plasmas has indicated that the gas is highly dissociated. This important prediction has only been recently confirmed by optical measurements, which require either external or internal calibration. Unfortunately, these optical techniques cannot readily be applied to the more chemically complex multicomponent plasmas commonly used in real processing. Using a new mass spectrometric method capable of detecting the local concentrations of both atomic and molecular atomic species within a plasma, a percent molecular dissociation of>85&percent;has been measured in an electron cyclotron resonance chlorine plasma at low pressures. ©1997 American Institute of Physics.