Porous spherical In(OH)(3) particles with controllable size were hydrothermally synthesized from aqueous In(NO3)(3) solution with the help of urea and citric acid. The average size of the synthesized In(OH)(3) particles decreased with the increase of the molar ratio between citric acid and In3+ concentration. Cubic In2O3 phase was obtained after a hydrothermal process at 160 degrees C and a calcination process at 600 degrees C, while mixed rhombohedral and cubic In2O3 phases were obtained by annealing the hydrothermally synthesized precursor at 180 degrees C. Morphology, internal structure, particle size distribution and specific surface area of the In2O3 particles were characterized. A possible mechanism was proposed to illustrate the growth of In2O3 particles. The gas sensors which prepared with porous cubic In2O3 materials displayed high sensitivity to formaldehyde. (C) 2016 Elsevier B.V. All rights reserved.