The first investigation of the ion chemistry of SeF6 and TeF6 is presented. Using a selected ion flow tube, the thermal rate coefficients and ion product distributions have been determined at 298 K for the reactions of fourteen atomic and molecular cations, namely H3O+, CF3+, CF+, CF2+, H2O+, N2O+, O+, CO2+, CO+, N+, N2+, Ar+, F+ and Ne+ (in order of increasing recombination energy), with SeF6 and TeF6. The results are compared with those from the reactions of these ions with SF6, for which the reactions with CF+, CF2+, N2O+ and F+ are reported for the first time. Several distinct processes are observed amongst the large number of reactions studied, including dissociative charge transfer, and F-, F, F2- and F2 abstraction from the neutral reactant molecule to the reagent ion. The dissociative charge transfer channels are discussed in relation to vacuum ultraviolet photoelectron and threshold photoelectron-photoion coincidence spectra of XF6 (X = S, Se, and Te). For reagent ions whose recombination energies lie between the first dissociative ionisation limit (XF6 → XF5+ + F + e-)and the onset of ionisation of the XF6 molecule, the results suggest that if dissociative charge transfer occurs, it proceeds via an intimate encounter. For those reagent ions whose recombination energies are greater than the onset of ionisation, long-range electron transfer may occur depending on whether certain physical factors apply, for example non-zero Franck-Condon overlap. From the reaction kinetics, limits for the heats of formation (in kJ mol-1) of SeF4, SeF5, TeF4 and TeF5 at 298 K have been obtained; ΔfH°(SeF4) < −369, ΔfH°(SeF5) < −621, ΔfH°(TeF4) > −570, and ΔfH°(TeF5) < −822.