The optical floating zone (OFZ) growth method was used to synthesize (Gd3-xREx)In2Ga3O12 (RE = Nd and Ho; x = 0; 0.15 and 0.30) single crystals. Initially, rare-earth oxide precursors Gd2O3 (Alfa Aesar, 99.99 %), Nd2O3 (Alfa Aesar, 99.99 %), Ho2O3 (Alfa Aesar, 99.99 %) were dried at 1200 °C for 12 hours, and In2O3 (Alfa Aesar, 99.99 %) and Ga2O3 (Alfa Aesar, 99.999 %) were dried at 700 °C for 12 hours. Feed rods were prepared by a conventional solidstate method using stoichiometric amounts Gd2O3, In2O3, Ga2O3 together with the respective activation amounts of RE2O3 (rare-earth oxide) according to the chemical equation (Gd3-xREx)In2Ga3O12. The powders were mixed in a planetary ball mill Retsch PM100 in ZrO2 grinding jars with a ball to powder ratio of 30:10 and a milling speed of 300 rpm for 24 hours, following calcination in an aluminium oxide crucible at 1500 °C for 8 hours in air. The assintered powders were re-grinded with 5 % excess In2O3 in order to compensate for the In2O3 evaporation and formed into cylindrical rods with diameter of 4 mm and a length of 100 mm in an isostatic press (70 MPa) using rubber forms and subsequently sintered at 1500 °C for 8 h in air. The rods were installed in an Optical Floating Zone Furnace (FZ-T-10000-H-III-VPR).