We report the observation of an anomalously large electro-optic coefficient in potassium titanyl phosphate (KTiOPO 4, KTP) waveguides fabricated under various conditions. An interferometric method was used to measure the effective electro-optic coefficient r 33 in the waveguides by measuring the phase retardation of an optical mode when a voltage is applied across the substrate. We observe that at low frequency the waveguide electro-optic coefficient can be higher than the bulk value by a factor up to about 100. Since KTP crystals are known to have a high concentration of mobile ions, a model relating the mobile charges, space charge fields, and observed enhancement in r 33 is proposed to explain the mechanism of this effect. Possible device application and long term effects on system operation are also discussed.