Observations aboard GEOS 1 and 2 show a correlation between generation of strong ion‐cyclotron ULF waves below the proton gyrofrequency and an increase in the cold He+ion concentration. The amplification process due to one cold species seems to be well understood and has been thoroughly discussed. Nevertheless, the amplification process when two cold species are present has not been so far properly studied. The GEOS experiments have also shown a close association between the occurrence of ion‐cyclotron ULF waves and the presence of thermal He+ions in the equatorial region of the magnetosphere (Young et al., 1979). Thus we study here the combined effect of cold H+and He+ions on the proton cyclotron electromagnetic instabil