Nanocrystalline thin films consisting of pure phases of a-GaO(OH), a-Ga 2 O 3 and b-Ga 2 O 3 have been prepared. These were synthesized by sol-gel technique through varying annealing temperature from 300 Β°C to 1100 Β°C in air ambient. Single phase a-GaO(OH) was obtained at annealing temperature of 300 Β°C. With increase in annealing temperature a mixed phase of a-GaO(OH) and a-Ga 2 O 3 was found. Pure a-Ga 2 O 3 could be prepared at 500 Β°C. As the annealing temperature was further increased, a-Ga 2 O 3 started to convert into b-Ga 2 O 3 . Only b-phase was observed at annealing temperature 700 Β°C and above. The average crystallite size of a-GaO(OH) and a-Ga 2 O 3 phases was found to be $3 nm and $16 nm respectively, while that of b-Ga 2 O 3 increased from $14 nm to $24 nm with increase in annealing temperature (700-1100 Β°C). The optical band gaps of a-GaO(OH) and a-Ga 2 O 3 , determined from transmittance measurements were 5.27 and 4.98 eV respectively and higher than that of b-phase. A nominal variation of band gap of b-phase (4.8-4.7 eV) was observed with the variation of annealing temperature (700-1100 Β°C). The structural defects of this phase, estimated from transmittance spectra using Urbach tail widths, were found to decrease with increase in crystallite size.