The fractional determination of L-dehydroascorbic (DAsA) and 2,3-diketogulonic acid (DKG) by the 2,3-dimercaptopropanol (PSH)-2,4-dinitrophenylhydrazine (DNP) method was investigated. DAsA, which reacts with DNP to produce hydrazone as well as DKG, is reduced to ascorbic acid by PSH. Excess of PSH is removed with N-ethylmaleimide, to prevent turbidity. Then DKG is incubated with DNP at 37℃ for 3 hours to make the hydrazone.