Nitrile oxides (3) are rapidly generated in situ on dissolution of the corresponding hydroxamoyl chlorides (1) in aqueous solution at pH > 4; reaction of (3) with the solvent is slow at pH < 10. Primary and secondary amines react stereospecifically to give only the Z-amidoximes (11), in which the nucleophile and OH groups are cis. With primary amines and ammonia the Z-isomer is also thermodynamically favoured but with the more bulky secondary amines subsequent isomerisation to the more stable E-isomer (12) occurs. Several criteria (ρ+0.53 for substituent variation in the nitrile oxide; βnuc 0.48 for primary amine variation; small enhancement shown by ‘α-effect’ amines; small steric effect of ortho-substituents in nitrile oxide) are consistent with a transition state for amidoxime formation in which there is little bond formation between the amine and the carbon of the nitrile oxide.