The formation kinetics of small oxygen clusters in hydrogenated Si has been studied by means of infrared absorption measurements. Hydrogen was introduced into the crystals by in-diffusion from H2 gas at 1200–1300°C. The samples were heated at temperatures in the range of 280–370°C for different durations. At initial stages of heat-treatment, enormous generation rates of the oxygen dimer have been observed in hydrogenated samples. This indicates highly enhanced diffusion of the interstitial oxygen atoms. The maximum achievable concentration of the dimers is found to be limited by their dissociation rate even at temperatures of about 300°C, while in as-grown crystals the capture processes are known to be dominant in this temperature region. An explanation of this phenomenon is presented.