Nuclear pure quadrupole resonance (n.q.r.) of the chlorine atom bound to carbon atom C(1) has been detected in several hexoand pento-pyranosyl derivatives. Observed resonance frequencies are distributed over two groups centred respectively at 32.53 and 3512 MHz. The structures of the two compounds which have the lower and higher frequency in the first group have been determined. The compounds are: tri-O-acetyl-6-deoxy-0c-e-mannopyranosyl chloride, C,207H17C1, (3310 MHz) and tetra-O-acetyl-~-D-mannopyranosyl chloride, Cz409H19Cl, (31.57 MHz). 1017 X-ray reflexions for the first compound and 1638 for the second were collected on an automatic counter diffractometer (Cu Ks radiation). A multisolution tangent refinement procedure was used for solving the structures. Values of R are respectively 45 and 7.5 %. The two compounds crystallize in space group P212a2~, Z=4. The lattice parameters are respectively: a=8.653(3), b=24.992(6), c=7.261(3) A and a=10.744(3), b= 22.264(7), e = 7833(3) A. The first compound adopts in the solid state the 1 C (L) conformation and the second the C 1 (D) conformation. In each case the C(1)-CI bond is axial. According to the particularly low value of the n.q.r, frequency, the C(1)-CI bond is significantly lengthened in the second compound, 1.856(8) ~, compared with the value of 1.82 ~ found in hexo-and pento-pyranosyl chlorides. The implications of this on the properties of the axial C-CI bond are discussed.