Thermal and dielectric studies of (ND4)(2)MoO2F4 crystals undergoing successive phase transitions at T-1 = 272 K and T-2 = 181 K showed that deuteration is accompanied by an increase in the chemical pressure in the crystal lattice (Delta p approximate to 0.02 GPa), which shifts the Cmcm ? Pnma transformation for the first order to the tricritical point. The direct participation of ammonium groups in the mechanism of structural distortions is demonstrated by a decrease in the entropy of the high-temperature phase transition (Delta S-1 = R ln 6.0). An external hydrostatic pressure leads to an expansion of the temperature interval of the intermediate antiferroelectric Pnma phase. The triple point on the T-p phase diagram, where the Cmcm, Pnma, amd Pnma* phases coexist, can be realized at a negative pressure of p(trp) approximate to -0.8 GPa.