Comparative studies of the effect of high pressures on the conductivity of ammonium halides NH4X (X = F, Cl, Br) at temperatures above 77 K are presented. High-pressure phase
transitions are observed which make themselves evident in a sharp change of the conductivity by several orders of magnitude at some critical pressures Pc and a hysteresis typical of the I kind
phase transitions. The Pc1 values correlate with the material density being approximately 40, 27 and 15 GPa for NH4F, NH4Cl and NH4Br, respectively.
The relaxation times of the conductivity with varying pressure are estimated. It is found that the relaxation times at pressures close
to the critical ones are much larger (hours to days) than at P > Pc1 (minutes).PACS: 81.40.Vw - Pressure treatment (see also 62.50.+p High-pressure and shock-wave effects in solids and liquids) 72.80.-r - Conductivity of specific materials (for conductivity of metals and alloys, see 72.15.-v) 72.60.+g - Mixed conductivity and conductivity transitions