When a dilute mixture of NH3 in neon is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 eV and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes three relatively prominent product absorptions which agree well with the positions determined in earlier gas-phase studies for the three infrared-active fundamentals of (NH3+)-N-14. The corresponding fundamentals of (NH3+)-N-15, (ND3+)-N-14, and (ND3+)-N-15 have been observed for the first time, as have been many of the fundamentals of the partially deuterium-substituted cations, in experiments on isotopically enriched NH3 samples. When the effects of anharmonicity are considered (including the occurrence of "negative anharmonicity" for the out-of-plane deformation fundamental), the positions of these absorptions agree satisfactorily with those predicted in an earlier study from the fit of an ab initio potential surface to the experimental data for (NH3+)-N-14, as well as with those predicted in the present study from a least-squares force constant adjustment to the frequencies observed for the various isotopomers.