Most plasmas used in the microelectronics industry are electronegative, i.e., they contain a large number of negative ions. One simple way to characterize the negative-to-positive ion density ratio(n−/n+)in such plasmas is to measure the velocity of ion acoustic waves (IAWs). In this letter, a detailed study of the propagation of IAWs in high-densitySF6magnetoplasmas is given. Results(n−/n+)obtained by way of this technique as functions of different parameters (gas pressure,SF6content inSF6/Armixture, and radial position in the reactor) are compared to those obtained with a more sophisticated technique based on laser photodetachment. ©1998 American Institute of Physics.