Psacalium decompositum (Gray) H.E. Rob. & Brett (Asteraceae), popularly known as "matarique," is a medicinal plant used in Mexico for the empirical treatment of diabetes mellitus. Previous studies with P. decompositum roots demonstrated that an aqueous fraction containing carbohydrates (WMP-fraction) exhibited a notable hypoglycemic effect in experimental animals. However, studies to identify these carbohydrates have not yet been performed. The goal of this investigation was to isolate and characterize the principal compounds in the WMP-fraction by spectroscopic and spectrometric analysis. Additionally, we sought to evaluate the hypoglycemic effect of these compounds in healthy and alloxan-induced diabetic mice. The results show that the chemical structures of the major compounds in the WMP-fraction correspond with fructan-type oligosaccharides and have a hypoglycemic effect in healthy and diabetic mice.