Explorations of the classical phase space for ion collisions with Rydberg atom targets provide insight into mechanisms for ionization and charge transfer. Thomas’s analysis of charge transfer at high impact energies [Proc. Roy. Soc. London114, 561 (1927)] is extended here to intermediate‐ and low‐energy ??? and ionization processes through numerical integration of the classical equations of motion. Parameter space maps correlating initial conditions with final outcomes have well‐defined zones corresponding to Thomas capture, direct capture, binary encounter ionization, saddle‐point ionization, and ionization bySsuperpromotion. The relative contributions to charge transfer (or ionization) cross sections from these distinct classical collision channels are qualitatively interpreted by analysis of the parameter space maps. ©1996 American Institute of Physics.