[en] Fast-slow mode conversion in deuterium plasma with a small amount of hydrogen impurity, for frequencies close to the two-ion hybrid frequency, is investigated. It is shown that while electron thermal effects tend to inhibit the wave conversion process, ion thermal effects tend to restore the cold plasma properties, favouring, therefore, the energy exchange between the two modes. The aforementioned effects are competitive for zeta₀sup(e) = c/n parallel to νsub(e) > approximately 1. For zeta₀sup(e) < 1, electron thermal effects, particularly Laudau damping, dominate over ion Larmor radius effects, drastically diminishing the wave conversion efficiency. For zeta₀sup(e) < < 1, the coupling between the modes disappears altogether. Furthermore, by studying the coupled equations describing the system, a coupling parameter is defined and shown to be the same in a plane slab geometry and cylindrical geometry. (author)