Transition-state theory based procedures for modeling the collision energy and internal temperature dependence of ion-molecule reactions are illustrated through a sample study of the O+(S-4) + CO2 reaction. Specific attention is paid to the effect of both short- and long-range interactions in the potential. Quantum chemical evaluations at the MP2/6-311G*+ level provide the data for a representation of the O+... CO2 bending potential at arbitrary separations. A variable reaction coordinate transition-state theory formalism is employed in an unsuccessful search for a short-range transition state with a reactive flux below that predicted by phase-space theory. However, the short-range bonding interactions are still important in providing an effective lower bound for the location of the transition state. A satisfactory description of the experimental data for this reaction is obtained via the incorporation of a constant intersystem crossing (or perhaps energy randomization) rate constant of about 1 x 10(11) s(-1) for the transition from a quartet to a doublet CO3+ complex. This intersystem crossing is a prerequisite to the production of the low-energy product O-2(2)+ + CO.