Ascorbic acid can be determined by titration with potassium ferricyanide in sulphuric, phosphoric or hydrochloric acid media. Twelve indicators, including ferroin and some triphenylmethane and thiazine dyes, have been found suitable. The method has been successfully applied to assay of commercial products for vitamin C. Several organic substances likely to be associated with ascorbic acid in preparations and plant tissues do not interfere.