The present invention relates to a novel process for preparing O-(3-amino-2-hydroxy-propyl)-hydroxymic acid halides of formula (I) by reacting a carboxamide oxime of formula (II) wherein R is as specified above with reactive 3-amino-2-hydroxy-propane derivative, diazotizing the O-substituted carboxamide oxime thus obtained with sodium nitrite in the presence of hydrogen halide, decomposing the diazonium salt and if desired, separating the optically active enantiomers and/or reacting the resulting base with an organic or mineral acid, wherein the carboxamide oxime of formula (II) is reacted with a 3-hydroxy azetidinium salt of formula (III) wherein R and R are as defined above and Y is a salt forming anion, in a lower alcoholic, preferably ethanolic medium optionally containing water and made alkaline with an alkali hydroxide, and before diazotizing the O-substituted carboxamide oxime intermediate obtained, the reaction mixture is neutralised and the organic solvent is removed. The process according to the invention provides the compounds of formula (I) with a higher yield compared to the prior art processes.