The separate reactions of the ²P_3/2 and ²P_1/2 ground states of Kr⁺ with O₂, CO, N₂O and OCS have been investigated over the mean relative kinetic energy (E_cm) range from 0.04 to several eV using a flow-drift tube (FDT) apparatus. The versatility of the technique has permitted state selective ion generation and depletion. Charge transfer has been observed from Kr⁺(²P_3/2) and Kr⁺(²P_1/2) to each neutral reactant with rate coefficients k and k* respectively. Although not previously observed at 300K, collisional Kr⁺(²P_1/2) to Kr⁺(²P_3/2) conversion (quenching) with the neutral reactant is taken into account, since this process can become significant at higher energies, as is demonstrated here in reactions with N₂O. This is described by a rate coefficient k_q. Experiments with CO suggested the reverse process of collisional conversion Kr⁺(²P_3/2) to Kr⁺(²P_1/2) also takes place above the necessary energy threshold. Experimental result (for the reactants shown) are presented for k(O₂, N₂O and OCS), k* (OCS) and k_t (N₂O, O₂ and OCS, where k*<k_f<k*+k_q) over the accessible E_cm range.