b ) 168, K 2 7 (1991) Subject classification: 76.30; S11.1 Institute of Problems in Material Science, Academy of Sciences of the Ukrainian SSR, Kiev') (a) and Duisburg University ( b ) E l e c t r o n Spin R e s o n a n c e of I r o n R h o m b i c C e n t e r s in K T a 0 3 A.P. PECHENYI ( a ) , M.D. GLINCHUK ( a ) , T.V. ANTIMIROVA ( a ) , and W. KLEEMAN (b) BY Fe3+ axial paramagnetic centers (PC) forming due to the nonisovalent substitution of lattice cations with charge compensation in nearest neighbouring position are widely used for an ESR study of phase transitions in KTa03 induced by dipole impurities /1/. Obtaining of reliable ESR information about the characteristics of the phase transitions and the nature of the low-temperature state of KTa03:Li (KTL) and KTa03:Nb (KTN) mixed crystals (which is widely discussed during recent time / 2 / ) is impossible without understanding of the structure of ESR spectra and the nature of the used paramagnetic probes. The present study is devoted to a detailed investigation of the rhombic ESR spectrum due to iron PC in KTaO in order to establish the nature of these centers, their charge state, lattice position, and the character of their nearest neighbouring.