The reaction of p -benzoquinone monosulfonate with sulfite ion was studied by DC and AC polarography and spectrophotometry and it was found that the reaction formed a highly reducing substance in alkaline solution. The redox potential of the substance is more negative than that of hydroquinone and the electrode reaction of the substance is reversible. Therefore, this substance can play a role in the hydroquinone development. The structure of this substance was concluded to be [III] or [III'](p.328); the form undergone an addition of sulfite ion to a quinone corbonyl group of p -benzoquinone monosulfonate.