The reagents carbazole – sulphuric acid, cysteine hydrochloride – sulphuric acid, have been applied successfully to the quantitative colorimetric determination of sedoheptulosan in pure solution or in the presence of ribose and/or fructose. Fructose can be determined quantitatively in the presence of sedoheptulosan and/or ribose by using a combination of the two reagents. Results indicate that while sedoheptulose reacts differently to sedoheptulosan with the orcinol reagent, the reactions with carbazole and cysteine are not altered by hydration and these may form a basis for the determination of the naturally occurring seven carbon sugar sedoheptulose.