The dispersion relation of an ion-acoustic wave propagating through a
collisionless, unmagnetised plasma, having warm isothermal electrons and cold
positive and negative ions has been derived. It is seen that the ion-acoustic
wave will be unstable in the presence of streaming of ions. Instability of the
wave is graphically analysed for the plasma having (H+,
O¯) ions, (H+, O2¯)
ions, (H+, SF5¯) ions,
(He+, Cl¯) ions and (Ar+,
O¯) ions with different negative ion concentration and relativistic
velocity.