The hemicellulose-derived aqueous solution of C6 and C5 sugars (D-mannose, D-galactose, D-glucose, D-xylose, Larabinose), obtained from the extraction and hydrolysis of the hemicellulosic polymers in softwoods prior to the production of pulp, was catalytically converted to the corresponding aldaric acids under alkaline (Pt/C) or native (Au−Pt/ZrO 2 ) aqueous conditions with air. The inhibiting effect of some residual impurities and degraded compounds in the hydrolysate was confirmed, such as colored unsaturated compounds and 5-hydroxymethylfurfural. A combined purification process consisting of filtration, demineralization by ion-exchange resins, evaporation, and active carbon treatment of the raw aqueous-stream gave a purified hydrolysate of aldoses that was oxidized to hexaric and pentaric acids. The yields were close to those of a synthetic solution of the pure sugars with the same distribution. The yields of aldaric acids accounted for ca. 50% of hexaric acids and up to 70% of pentaric acids.