Electron paramagnetic resonance (EPR) has been used to identify the primary electron and hole traps responsible for ‘‘gray tracks’’ in flux‐grown KTiOPO4(KTP). Ionizing radiation (x rays) was used to produce the gray‐track effect. During an irradiation at 0 °C, a broad absorption band peaking near 500 nm is introduced, the EPR spectra from a series of Ti3+centers appear, and the dominant EPR spectrum associated with Fe3+ions decreases significantly. Following the irradiation, the decay of the optical absorption and the Ti3+centers, along with the growth of Fe3+centers, were monitored over a period of 20 h at room temperature. Changes in the EPR spectra of the Ti3+and Fe3+centers during the anneal correlated with the decay of the induced optical absorption (i.e., gray track). These results demonstrate that Fe3+centers are the primary hole trap and Ti4+‐VOcomplexes are the primary electron trap responsible for gray track formation in flux‐grown KTP crystals. ©1995 American Institute of Physics.