The solid-solution compounds of Gd-4(Al1-xGax)(2)O-9 (x = 0.0-1.0) were prepared at 1600 degreesC for 5 h in air. The unit cell volume of the compounds increased from 0.853 to 0.878 nm(3) with x. Phase transitions having a temperature hysteresis were observed from 1100 degrees to 1400 degreesC by calorimetry and dilatometry. The transition temperature increased with x. The volume of the high-temperature phase was 0.5% smaller than that of the low-temperature phase at the transition temperature. The volume changes were independent of x. The hysteresis width observed by the dilatometry was about 300 degreesC for the Gd4Al2O9 ceramics (grain size: about 1 mum) and decreased to 50 degreesC for the Gd-4(Al0.2Ga0.8)(2)O-9 ceramics (grain size: over 10 mum). Gd4Ga2O9 was unstable at low temperature and decomposed to Gd3GaO6 and Gd3Ga5O12 during the thermal analyses.