Abstract <p>Ion motion upon propagation and collisions of ion-acoustic solitary waves in the electron–ion plasma and in the negative-ion plasma is analyzed. Ion displacement after wave propagation is calculated for a number of cases, and its dependence on the wave amplitude is found. The back action of the processes that occurs in plasma upon collisions of the ion-acoustic solitary waves on the waves themselves is described. Physical mechanisms that prevent preservation of the wave identity upon their mutual collisions are discussed. The influence of ion displacement on formation of caustics upon collisions of the ion-acoustic solitary waves in the negative-ion plasma is analyzed.</p>