  A general first-principles theory of dissociative recombination is developed
for highly-symmetric molecular ions and applied to H$_3$O$^{+}$ and CH$_3^+$,
which play an important role in astrophysical, combustion, and laboratory
plasma environments. The theoretical cross-sections obtained for the
dissociative recombination of the two ions are in good agreement with existing
experimental data from storage ring experiments.
