When measuring the rf emission spectra of an electron cyclotron resonance plasma by a shielded antenna under different discharge conditions, a parametric decay of electron cyclotron waves was detected. Above a certain threshold of the launched microwave power, the pump wave at&ohgr;0/2&pgr;=9.733 GHzdecays into two different wave modes following the condition&ohgr;2=&ohgr;0±&ohgr;1,where&ohgr;1is in the ion cyclotron resonance frequency range. Thus heating of ions is possible. Light ions are preferably heated. We consider this mechanism a possible candidate for explaining the isotope anomaly detected by A. Drentje &etal; [Rev. Sci. Instrum.67, 3 (1996)], which indicates that the heavier isotope of the same ion species is preferentially enriched in the discharge, whereas the lighter one is more easily transported out of the plasma. Experimental results are discussed with respect to this hypothesis. ©1998 American Institute of Physics.