Measurements of the temperature dependence of the conductivity of single crystals of ammonium dihydrogen phosphate are described. Impurities introduced in known amounts as substituents in the lattice enable the activation energy for mobility to be separated from the total energy in the Boltzmann factor for conduction. With Ba++as impurity, the activation energy is 10.1 kcal/mole, with SO4−−10.9; the average, 10.5 kcal/mole, is taken to be the activation energy for mobility in this crystal. It ls proposed that activation for mobility in this solid is due to the simultaneous breaking of two hydrogen bonds independently. The &sgr;0factors, both for intrinsic conduction and for impurity conduction, agree reasonably well with the values expected from the theory of ionic conduction. Measurements on crystals enclosed in an evacuated space show that hydrogen gas is generated by the electrolysis of the crystal. According to Faraday's law the theoretical yield for a univalent ion leading to a diatomic molecule (H2) as product is 0.5 mole/F; the observed yields ranged from 0.3 to 0.7 moles/F. It is inferred that the conduction is ionic and that the migrating particle is probably the proton. From the conduction data for the pure crystal, the energy required for the formation of ions within the crystal is calculated to be 19.8 kcal/mole. The mobility process in this crystal shows a strong resemblance to that in ice. Both appear to be referable to the breaking of two hydrogen bonds simultaneously and to the tunneling of protons between equivalent sites. Both crystals have hydrogen bonds as an important part of their structure.