Hydroxamic acids and their anions exhibit geometrical isomerism due to amide-like resonance and an increase in the C-N bond order which can be monitored by H-1-NMR spectroscopy. N-Phenylpropiono- and dodecanohydroxamic acids, la and 2a, respectively, exist as E-isomers in DMSO-d(6) and CD3OD but la is an E-Z mixture in D2O. The corresponding hydroxamate ions, Ib and 2b, also exist as E-isomers in DMSO-d(6), but Z-isomers form on addition of D2O and Z-2b is dominant with chi(D2O) greater than or equal to 0.9. In CD3OD E- and Z-isomers coexist, but Z-2b becomes dominant on addition of D(2)0. The E:Z ratio of Ih is very similar in cetyltrimethylammonium bromide (CTABr) and in D2O, but only Z-2b is detected in cationic CTABr micelles and in anionic, zwitterionic, and nonionic micelles. Formation of Z-2b in micelles and water-rich mixed solvents is favored by hydrophobic interactions between phenyl and n-alkyl groups, despite the proximity of anionoid oxygens. In micelles of sodium dodecyl sulfate the N-phenyl residue of 2b is in a more aqueous region of the micelle-water interface than in micelles of the other surfactants.