The stability of ion Bernstein waves (frequency ω, wave number ) in a plasma with two ion species pumped by a magneto-acoustic wave (ωp, p) which propagates perpendicularly to the static magnetic field is studied. The background plasma is assumed to be infinite, homogeneous and collision-free. If ωp > |ω| is properly chosen, ion Bernstein waves become unstable (decay instability, non-linear Landau instability) at rather low values of the pump electric field amplitude |D|. The instability is excited by the relative drift motion of different species induced by the pump wave. Assuming |⋅(a−b)|≪1 (a = displacement for particles of species a, a = e, 1, 2) the general non-linear dispersion relation is approximately solved in a deuterium-tritium plasma for two different : In case A (=⊥ ∥ p) only the relative ion motion d⋅t comes into play; this gives rise to decay instabilities which are only present in plasmas with two ion species. The instabilities of case B(k⊥ ⊥ kp, kz ≠ 0) caused mainly by the relative electron-ion drift motion are similar to those in plasmas with only one ion species. For given |p| the growth rates in both cases are equal in order of magnitude (for low values of ωp, case A is slightly more favourable); however, in case A the theory is valid up to considerably higher values of |p|. Some effects depending substantially on the presence of two ion species are discussed in detail.