Evidence supporting the temperature dependent defect mechanism of nonstoichiometry on the potassium and oxygen sublattices in KTP is presented. The primary compensating defects for the formation of vacant potassium sites in typical flux grown KTP are vacant oxygen sites. Protons (OH-) are the principal defect compensating for the formation of vacant potassium sites in high temperature hydrothermal KTP. A model of the ionic conductivity in high temperature hydrothermal KTP is proposed in which specific protons participate in cooperative motion over a limited distance with the potassium vacancies migrating along the “channels” in the structure in the Z-direction. The higher activation energy measured for ionic conductivity in flux grown KTP (0.5 eV) relative to high temperature hydrothermal (0.3 eV) is suggested to be due to the energy required to dissociate from a defect complex, such as a (VO - VK). The correlation of ionic conductivity to damage susceptibility appears to be due to the levels of compensating defects for vacant potassium sites in KTP, which are related to the concentrations of Ti3+ formed in the crystals. Further study is ongoing to understand the specific mechanisms involved in the ionic conductivity and damage in KTP grown by the flux and hydrothermal techniques.