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-density SF6 magnetoplasmas is given. Results (n−/n+) obtained by way of this technique as functions of different parameters (gas pressure, SF6 content in SF6/Ar mixture, 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.