[en] The reactive scattering of the reaction of the first excited state of C⁺ (⁴P) with molecular hydrogen was studied over an energy range of 0.5 to 9.0 eV. The measurements indicate that the reaction proceeded by a direct mechanism and the final product was in the a³pi state. The reaction of ground-state C⁺(²P) with D₂ and H₂ was examined over the energy range of 2.5 eV to 28 eV. Over this energy range unambiguous identification of CD⁺ in the X¹sigma, a³π and A¹π states was demonstrated. In addition the vibrational structure in the energy spectrum of CH⁺ and CD⁺ was resolved for the first time; the v = 0, 1, 2, and 3 product vibrational states were populated in the relative ratios 1.0:0.52:0.21:0.04. The vibrational band shapes permitted a determination of the rotational level distribution in the product. Careful measurements of the limiting value of the translational exoergicity (Q/sub MIN/) showed that the product state in the reaction F⁺(H₂,H)FH⁺ is ²π and that no FH⁺ in the ²sigma⁺ is formed although energetically feasible. Furthermore, it was shown that Q/sub MIN/ for the reactions of F⁺ with H₂ and D₂ were significantly lower than the values expected from known thermodynamic data. Experimental measurements coupled with quantum mechanical calculations showed that the displacement of Q/sub MIN/ in the reaction F⁺(H₂,H)FH⁺ and the corresponding deuterium analog relative to the thermodynamically expected value arises because of the presence of a dynamic rotational barrier in the potential energy curve. The reactions of Cl⁺(³P,¹D) with D₂ were studied in the energy range 1 to 100 eV. Energy and angular distributions of the DCl⁺ ions were measured. The reaction was found to proceed by a direct mechanism, and in both reactions the product was in the ²π state. Energy transfer processes involving inelastic and superelastic collisions of Cl⁺(³P) and Cl(¹D) with D₂ and Ar were also briefly studied