The effect of the thulium (Tm) substitution in the Tb(3)AI(5)O(12), Tb3-xTmxAl5O12 (TTAG) mixed garnet crystals, has been studied by the solidification of corresponding stoichiometric melts using a micro-pulling down apparatus. The phase composition for the TTAG crystals with different TM3+ concentrations was determined by powder X-ray diffraction analysis. The amount of the TbAlO3 perovskite phase in the as-grown TTAG fibers was observed to decrease with substitution of Tm3+ in Tb3+ site. The lattice parameters of the TTAG were reduced with increase of concentration of Tm3+. The results of the Verdet constant measurements are also reported. (C) 2003 Elsevier B.V. All rights reserved.