CH3NO 2, Mr= 61.04, monoclinic, Cc, a = 3.636 (3), b=9.745(2), c=7.512(3)A, fl= 116.56(4) °, V= 238.1 (2) A 3, z = 4, D m (293 K) = 1.64, D x = 1.703 Mg m -3, 2(Mo Ka) -- 0.71073 A, g = 0.154mm -1, F(000)=128, T=105K, final R= 0.030 for 995 unique observed reflections. The conformation of O=CH-NH-OH is synperiplanar (sp) with an O=C-N--O torsion angle of 5.4 (4) °. The crystals are stabilized by a network of intermolecular hydrogen bonds. Data for a series of hydroxamic acids, RCONHOH, and hydroxamate ions and ligands have been retrieved from the Cambridge Structural Database. Bond distances and bond angles as well as conformational parameters of the hydroxamic acid moiety have been analyzed. The antiperiplanar (ap) conformation of the O=C-NH-OH moiety is found to be almost as common in crystals as the sp conformation. Only one case of intramolecular OH...O bonding in sp conformers was found. Variations in C=O and C-N bond lengths are correlated. Charge delocalization through the entire hydroxamate moiety is observed in the ligands, but not in the ions. Opening of the O=C-N angle is most pronounced in sp conformers of hydroxamic acids. In hydroxamate ligands this angle is less than 120 ° .