The invention relates to a novel process for producing the threo- and erythro-isomers of 1-phenyl-2-nitro-1,3-propanediol through the aldol-type alkaline condensation of benzaldehyde and nitroethanol, wherein the reaction mixture is acidified and, if desired, the isomers are separated from the obtained isomer mixture and the obtained isomers are transformed into each other through epimerization, characterized in that the condensation and, if desired, also the epimerization are carried out in the presence of catalytical amounts of an alkaline hydroxide.The process according to the invention makes it possible to produce 1-phenyl-2-nitro-1,3-propanediol, an intermediate of the antibiotic chloramphenicol, in high yields, and in an extremely advantageous way which can be carried out simply and economically even on an industrial scale.