The microbiological oxidation of 1-deoxy-1-S-ethyl-D-arabitol by Acetobactersuboxydans has yielded syrupy 5-deoxy-5-S-ethyl-D-threo-pentulose from which a crystalline phenylosazone identical with the one prepared from 5-deoxy-5-S-ethyl-D-xylo-pentose was obtained. A syrupy 3,4-O-isopropylidene-5-deoxy-5-S-ethyl-D-threo-pentulose and its crystalline 2,5-dichlorophenyl-hydrazone were prepared.