Electron paramagnetic resonance (EPR) is used to characterize Pt+ (5d(9)) ions in flux-grown single crystals of KTiOAsO4 (KTA). These platinum ions are unintentionally present. In the as-grown crystals, Pt0 (5d(10)) atoms substitute for K+ ions. When a crystal is irradiated with X-rays at room temperature or 77 K, the Pt-0 atoms trap holes and convert to Pt+ ions. Once formed, these Pt+ ions are stable for weeks at room temperature. The EPR spectrum of the Pt+ ions is best observed in the 20-30 K range. Principal values and principal-axis directions for the g matrix and the Pt-195 hyperfine matrix are obtained from EPR angular dependence data taken in three crystal planes. These principal values are 1.509, 1.898, and 2.859 for the g matrix and 96, 317, and 616 MHz for the Pt-195 hyperfine matrix. Formation of the Pt+ ions is accompanied by a gray coloration that is expected to affect device performance when KTA crystals are used in high-power nonlinear optical applications. As-grown KTA crystals with larger concentrations of platinum may have lower ionic conductivities, as Pt-0 atoms replace potassium vacancies. (C) 2008 Elsevier B.V. All rights reserved.