X-ray flow visualization in a half-zone molten silicon bridge was carried out under microgravity at a variety of oxygen partial pressures in an ambient atmosphere. The flow velocity of thermocapillary convection was found to decrease with increased oxygen partial pressure. Thermocapillary flow stabilized when the oxygen partial pressure at the inlet P-O2 was increased to 10(-4) MPa. The dependence of the flow velocity and flow mode on p(O2), can be explained in terms of the decrease in the temperature coefficient of surface tension due to the increase in oxygen partial pressure.