Oxidase reagent consists of either N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride or p-aminodimethylaniline in distilled water.
The chemical serves as the artificial final electron acceptor from the terminal cytochrome oxidase, turning from clear to purple/indigo blue when reduced by the added electrons. Bacteria lacking cytochrome oxidase cannot add electrons to this chemical, and their colonies do not change color. For the software, the oxidase test is performed using paper strips impregnated with the reagent.