A nonlinear process is considered where a tranverse wave with a frequency near the plasma frequency decays into a plasma oscillation and an ion acoustic wave. The inverse processes are investigated. Also, the interaction of four waves—one tranverse, one ion acoustic, and two plasma oscillations are investigated. The energy of the tranverse wave is partially converted to that of the ion acoustic wave in the presence of a plasma oscillation in both cases. A necessary condition for the process of four wave interaction is obtained. The phases of all interacting waves are assumed to be perfectly matched.