The ′ Rydberg states of the ammonia isotopomers NH₃, NH₂D, NHD₂ and ND₃ have been investigated by (2 + 1) resonance enhanced multiphoton ionisation spectroscopy. The spectroscopic parameters for the zero point and v^′₂ = 1 vibrational levels of the ′ ¹A^′₁ state have been derived for the first time for the mixed isotopomers. The current work provides the basis for future studies of the chemical reactions of the mixed isotopomers of ammonia ions formed by the REMPI technique.