We report on optical and electrical characterisation of InN thin films prepared by RF reactive sputtering of In target with pure nitrogen gas. The resistivity of the films is in the range of 10(-3) - 10(-2) Ohmcm, the mobility as high as 306 cm(-2) V-1 s(-1) and the carrier concentration is typically in the order of 10(19) cm(-3). Optical measurements show that films are highly degenerate with band gap values within 2.0-2.1 eV. Aging and annealing treatments indicate that physisorbed oxygen is eventually chemisorbed into the InN films, converting them into an oxynitride (InON) phase. This is observed through the increase of the band gap energy by about 0.2 eV in the samples aged for 6 months and an even greater increase of 0.8 eV in the samples annealed at 400degreesC. (C) 2002 Published by Elsevier Science B.V.