Structural determination of the isomeric L-arabinosylureas (1a, 2a, and 3a) and their facile isomerization into the thermodynamically stable isomer (3a) by acid are described. The reaction of L-arabinose with 3-methoxy-n-propylamine followed by treatment with 2-chloroethyl isocyanate gave a mixture of the ureas (1a, 2a, and 3a) in a ratio of 3 : 2 : 1. They were determined to be the β anomer (1a) and the α anomer (3a) of 3-(L-arabinopyranosyl)-1-(2-chloroethyl)-3-(3-methoxy-n-propyl) urea and the β anomer (2a) of the corresponding L-arabinofuranosyl derivative, respectively. Isomerization of these ureas was examined in formic acid solution by monitoring their thin layer chromatography. Complete isomerization of both 1a and 2a into 3a was observed, while 1a isomerized into 3a via 2a. Consequently, the selective isomerization of a mixture of these ureas into 3a could readily be effected by formic acid treatment. The nitrosation of these ureas by the use of dinitrogen tetroxide gave the corresponding nitrosoureas (4a, 5a, and 6a), and their structures are discussed.