The peracetylated diethyl dithioacetals of d-ribose, d-arabinose, d-xylose, d-lyxose, l-rhamnose, and l-fucose were studied in chloroform- d solution by n.m.r. spectroscopy at 100 MHz. Signals of all protons were assigned, assignments were verified by spin-decoupling, and first-order coupling-constants were determined. For the d-arabinose, d-lyxose, l-rhamnose, and l-fucose derivatives, the coupling constants observed accord with a favored conformation in which the chain of carbon atoms is fully extended in a planar, zig-zag arrangement. The d-ribose and d-xylose derivatives, which, in a conformation having the carbon atoms in a planar, zig-zag arrangeent, would have a pair of parallel, eclipsed acetoxyl groups at C-2 and C-4, give coupling data completely inconsistent with such an extended, planar formulation. The data indicate that the favored conformation of the d-ribose and d-xylose derivatives is a “sickle” arrangement derived from the planar, zig-zag conformation by rotation (about the C-3C-4 bond in the d-rihose derivative, and about the C-2C-4 bond in the d-xylose derivative) to rotamers in which the steric interaction between substituents at C-2 and C-4 is relieved.