The effect of refining agents in soda-lime-silica melts was examined by measuring the oxygen activity with using electromotive force. When the melt contained antimony, the activity changed with temperature and the variation in activity increased as the antimony content increased. When the oxygen content in the atmosphere changed, the oxygen activity in the melt changed gradually by diffusion. The variation in oxygen activity with atmosphere decreased with increasing antimony content and almost disappeared when the Sb203 content was 1 mol%. In the case of sulfate addition, the activity value became unstable when the temperature was raised, which might be caused by bubble formation accompanied by the decomposition of sulfate.