The crystal structure of merosinigrin [(1′S,2′S)-1,2-S, O-(1-cyano-2-methyl-1,2-ethanediyl)-1-thio-β-d-glucopyranose], the long-known rearrangement product of allylglucosinolate, was determined by X-ray diffraction at 90 K, thereby revealing the geometry of an (ap,ap)-monothioacetal group. The crystals of merosinigrin, C₁₀H₁₅NO₅S, are triclinic, P1, with cell dimensions a = 5.349(2), b = 6.664(3), c = 9.266(4) Å, α = 85.91(3), β = 76.35(3), γ = 70.83(3)°, Z = 1. The structure was solved by direct methods and refined to R = 0.035 for 1789 independent reflections. For thioglycosides and other monothioacetals, the lengths of the central C-S and C-O bonds depend on the torsion angles about these bonds, consistent with a simple electron-delocalisation model. The central S-C-O valence angle in monothioacetals also Depends in a regular way on the torsion angles. Energy optimisation of methoxy(methylthio)methane, a thioglycoside model, by semiempirical molecular orbital calculations (MNDO) reproduced trends observed in the experimental data.