We succeeded in growing gray-track-resistant KTP single crystals in a KTP-K6P4O13(K-6)-PbO system using TSSG (top seeded solution growth) and have explored the mechanism of gray track formation using combined characterization technologies. The optical and laser properties of the crystals were estimated. To obtain a better understanding of the gray track effect, in situ X-ray absorption near edge structure (XANES) measurements using the total electron yield (TEY) method in the region of gray track formation was performed. Electron paramagnetic resonance (EPR) was used to further confirm the existence of Ti3+. The results indicate that the formation of the gray track is related to Ti3+ centers. (C) 2014 Elsevier B.V. All rights reserved.