Single crystals of ruthenium silicide Ru2Si3 have been grown by the floating zone technique with radiation heating in order to investigate the thermoelectric efficiency of this compound. Special attention was paid to the crystal perfection and the mechanism of defect generation, which may cause the deterioration of thermoelectric properties. Crystallographically oriented plate-like silicon inclusions in the Ru2Si3 matrix, about 1 mum wide, have been detected. They are supposed to be a result of solid-state precipitation during cooling at temperatures below 1000degreesC. Annealing at 1000degreesC for 24 h with subsequent freezing leads to the disappearance of these defects. The resistivity, Hall and Seebeck coefficients of single crystals have been measured across a wide temperature range. Undoped crystals show both types of conductivity in the thermoelectric power. At T = 745 K the Seebeck coefficient reaches its maximum value of 300 muV/K. The room temperature carrier concentration in undoped material is about 10(18) cm(-3). The Hall mobility at room temperature is 2-3 cm(2)/V s. The hole mobility calculated from Hall data varies with temperature as T-3/2, indicating a predominant scattering of carriers at ionized impurities. The complete absence of acoustic phonon scattering allows the conclusion that point defects in the crystals dominate the intrinsic lattice properties. The intrinsic conductivity sets on at 590 K, which is a relatively low value for refractory wide gap silicides. This confirms the high quality of the crystals. (C) 2002 Elsevier Science B.V. All rights reserved.