In(OH)(3) and InOOH were synthesized in a high-temperature continuous flow microreactor, which was much faster than the hydrothermal synthesis in a Teflon-lined autoclave. The phase transition interval of In(OH)(3) and InOOH is measured, and the effect of aging temperatures on equilibrium compositions was also theoretically calculated. A transformation from nanorods to nanocubes was observed when the aging temperatures increased. In(OH)(3) and InOOH were also synthesized with Sn dopant, which was proven to be beneficial for the transformation from In(OH)(3) to InOOH. The sensors based on cubic and hexagonal In2O3 particles showed a fast response (4-5 s) and recovery speed (12-15 s) to acetone vapor at the optimum operating temperature of 290 degrees C. The sensor based on hexagonal In2O3 showed a higher response than that based on cubic In2O3. This work provided a rapid, continuous, and high-temperature synthesis method of an In(OH)(3), InOOH, and In2O3 series.