A method for the oxidation of substrates comprising treating an aqueous, basic solution of a substrate having an oxidizable functionality using an elemental halogen as terminal oxidant in the presence of an oxoammonium catalyst/halide co-catalyst system. Use of elemental halogen, preferably chlorine gas or elemental bromine, unexpectedly allows oxidation without significant degradation of the substrate. The substrate is preferably a monosaccharide, oligosaccharide, or polysaccharide, and the oxidizable functionality is preferably an aldehyde, hemiacetal, or a primary alcohol. An effective source of the oxoammonium catalyst is 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and a particularly economical and effective catalyst is 4-acetylamino-2,2,6,6-tetramethylpiperidinyl-1-oxy.