The impact ionization of atomic hydrogen in collisions with H +, He 2+, Li 2 , 3 + , C 2 , 3 , 4 , 5 , 6 + , N 2 , 3 , 4 , 5 + , O 2 , 3 , 4 , 5 , 6 + , and Ar 3 , 4 , 5 , 6 + ions has been studied by the modified over-barrier model at low-to-intermediate velocities. Compared with the calculated results of the classical trajectory Monte Carlo (CTMC) method and the unitarized-distorted-wave approximation (UDWA), this model satisfactorily reproduces the experimentally obtained velocity dependence of the absolute ionization cross sections at intermediate impact velocities.