  We study the sensitivity of the microwave and submillimeter transitions of
the isotopologues of hydronium to the variation of the electron-to-proton mass
ratio mu. These sensitivities are enhanced for the low frequency mixed
inversion-rotational transitions. The lowest frequency transition (6.6 GHz)
takes place for isotopologue H2DO+ and respective sensitivity to mu-variation
is close to 200. This is about two orders of magnitude larger than the
sensitivity of the inversion transition in ammonia, which is currently used for
the search of mu-variation in astrophysics.
