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 14 atomic and molecular cations, namely, H3O+, CF3+, CF+, CF2+, H2O+, N2O+, O+, CO2+, CO+, N+, N-2(+), 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 among the large number of reactions studied, including dissociative charge-transfer and F-, F, F-2(-), and F-2 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, or Te). For reagent ions whose recombination energies lie between the first dissociative ionization limit (XF6 --> XF5+ + F + e(-)) and the onset of ionization 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 ionization, long-range electron transfer may occur depending on whether certain physical factors apply, for example, nonzero 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: Delta H-f degrees (SeF4) < -369, H-f degrees (SeF5) < -621, H-f degrees (TeF4) > -570, and Delta H-f degrees (TeF5) < -822.