PURPOSE: To determine the concentration of the reduced type vitamin C in a sample accurately in a short time by a simple method by supporting 2,6-dichlorophenolindophenol in insoluble polyvinylpyrrolidone.

CONSTITUTION: The 2,6-dichlorophenolindophenol (DCP) is supported by the insoluble polyvinylpyrrolidone. Various types of the reduced type vitamin C wherein the DCP is supported by the PVP are possible. For example, the PVP including the DCP is enclosed in a capillary, or the PVP including the DCP is applied on tacky tape or kneaded to filtering paper. In determining the reduced type vitamin C, one end of the determining reagent is dipped in the sample liquid, the sample liquid rises up by the capillary phenomenon, discoloring region of the reagent is obtained, and the concentration of the reducing type vitamin C in the sample is computed from a standard curve.