A novel low-temperature (<50 degrees C) oxidation process of a a-Si:H thin film in a low-energy ( approximately 30 eV) RF hollow oval magnetron system was studied. SiH4 and O2 flow rates were alternatively modulated in a low-pressure (4mTorr) argon discharge, and oxygen pressure reduction was monitored in situ to investigate the oxidation right after the deposition of ultrathin a-Si:H film with various thickness (<100 AA). The oxidation is mainly promoted by the ion-enhanced reaction and diffusion of neutral oxygens. It is led by a high-rate reaction-limited region in the top 40 AA with oxygen reactivity near unity, and followed by a diffusion-limited region with decreasing oxygen consumption rate. The diffusion rate becomes very low beyond the top 80 AA. O-H and Si-H modes in the IR spectrum were observed for the substoichiometric film and disappeared when the film was fully oxidised.