We have reported the taxonomy, production, isolation and preliminary characterization of new mannosidase inhibitors, mannostatins A and B. Here we report the structure determination of mannostatins A and B. Mannostatin A was obtained as a colorless syrup, positive to ninhydrin, naphthoresorcinol - sulfuric acid and anthrone reactions, with molecular formula C₆H₁₃NO₃S from HR-MS. Through ¹H NMR, ¹³C NMR, spin decoupling experiments, and tetraacetate derivative analysis (showing one amino and three hydroxyl groups), its structure was confirmed to be 4-amino-5-methylthio-1,2,3-cyclopentanetriol. Mannostatin B has similar chemical properties to A; ¹H NMR, ¹³C NMR, tetraacetate derivative analysis (molecular formula C₁₄H₂₁NO₈S), and conversion to A by thioglycolic acid treatment indicated it is the sulfoxide derivative 4-amino-5-methylsulfinyl-1,2,3-cyclopentanetriol. The configuration of mannostatin B was determined by X-ray diffraction analysis of its tetraacetate (crystal data: monoclinic, space group P2₁, Z=2, Dcalc=1.359 g/cm³; solved by direct method, refined to R=0.058), showing a distorted half-chair cyclopentane ring, specific bond lengths/angles/torsional angles, substituent configurations (confirmed by anomalous dispersion corrections for absolute configuration), and hydrogen bonding with three hydration water molecules in the crystal. Mannostatins A and B have unique cyclopentanol structures containing methylthio or methylsulfinyl groups, different from other glycosidase inhibitors.