For the first time, reducing values of homologous series of oligosaccharides have been determined by the 2,2'-bicinchoninate assay. The extent of Cu2+ reduction was monitored by spectrophotometric measurement of Cu+/2,2'-bicinchoninate complexes. Conditions of the assay were optimized so that relatively uniform reducing values were obtained with oligosaccharides derived from starch, polygalacturonic acid, and chitin, regardless of degree of polymerization. The uniform values resulted because members of each oligosaccharide series were oxidized to similar levels beyond the aldonic acid stage, while oxidation was limited to the reducing-terminal residue. This conclusion was based on direct measurements of quantitative loss of paramagnetic copper (Cu2+) by electron paramagnetic resonance spectroscopy.