Selective monooxidation of symmetric dialdehydes in aqueous media have been investigated for several symmetric dialdehydes. The best conditions apply 1.1 equivalents of H5IO6 with 0.02 equivalents of a co-oxidant PCC in H2O containing CH3CN as a co-solvent (H2O/CH3CN = 10/1). The reaction conditions are simple, mild, and practical, yielding the corresponding formyl carboxylic acids in modest to high yields.