Numerical solutions of the Vlasov and Poisson equations for the expansion of a plasma with O+and H+as the constituent ions are presented. The self‐similar nature of the expansion is examined by comparing the numerical results with available theories. The formation of a plateau in the expansion region, when the H+ions are initially minor, is investigated by varying the electron to ion temperature ratio. The numerical results show that self‐similarity holds in the quasineutral regions of the plasma.