The effect of thermal ions on the convective properties of the electromagnetic proton cyclotron instability is investigated. It is shown that when Ap < 1, where Ap is the proton anisotropy, cold ions, with relative specific charge mj/Zj mp ≥ 1, stabilize the convective growth of the instability. When Ap >1, the effect of thermal ions is to enhance the convective growth of the instabifity in the range where the real part of the frequency, ωr, is smaller than the ion gyrofrequency, Ωj. The maximum enhancement, corresponding to an optimum concentration of heavy ions, is shown to be a constant independent of the species. However, for each species, even for Ap > 1 (except for protons), there is always a range of values of Ap such that the species will stabilize the system. Heavier ions are shown to be more effective in enhancing the convective growth of the instability, in the sense that, the heavier they are, smaller relative concentrations are required to reach the constant value mentioned above. For sufficiently heavy species, the required concentration to optimize the convective growth, is independent of the species.