The marginal‐stability boundary for spontaneous generation of ion‐acoustic waves in a nonequilibrium plasma is investigated experimentally. The transition from inverse Landau damping to normal Landau damping of the acoustic wave modes is demonstrated by observing the change in forward‐scattering cross section of microwave electromagnetic energy. The measured scattering cross section is compared with that predicted by Ichimaru, using a simple relaxation‐time collision term in the Boltzmann equation. In addition, the collision term of Bhatnagaret al.is used to construct a new marginal‐stability boundary. The latter expression seems to give a better comparison with the observed microwave scattering and demonstrates the superiority of the more realistic collision‐term model.