Using the KBM perturbation technique, the stability of oblique modulation of ion‐acoustic waves in a two‐ion plasma is studied. It is found that the presence of a small amount of lighter ion impurities significantly changes the instability domain in the &ohgr;‐&fgr; plane. The effect of the concentration and mass of impurity ions on the modulational instability is discussed in detail. The threshold amplitude for instability and nonlinear frequency shift of the wave are also calculated. The predictions of the theory are found to be in good agreement with the experimental observations.