Vapor pressures as a function of temperature of the ammonium salts are measured by means of a simultaneous torsion effusion and mass-loss effusion technique. From a theoretical analysis, which is given first, it is shown that the enthalpies of sublimation and the degree of dissociation b in the vapor phase are related to the measured quantities. The degree of dissociation appears to be a weak function of temperature and is evaluated by b(NH4F, 288.91 K)=0.97; b(NH4Cl, 352.02 K)=0.85; b(NH4Br, 380.06 K)=0.51; b(NH4I, 385.03 K)=0.39; b(NH4CNS, 313.78 K)=0.61; b(NH4NO3, 351.89 K)=0.66; b(NH4HCO3, 270.56)=0.85. It is assumed that NH4HCO3 dissociates into NH3, H2O, and CO2. At the given temperatures, total vapor pressure is 0.40 Pa.