We study the peroxidase-oxidase reaction with new experimental methods useful for the deduction of oscillatory mechanisms. We measure four species, oxygen, reduced nicotinamide adenine dinucleotide (NADH), native horseradish peroxidase (HRP), and compound III (coIII), simultaneously to determine their roles in the reaction mechanism. We measure the relative amplitude and phase shift relations of these species and perform concentration shift regulation and destabilization, qualitative pulsed-species response, and quench experiments. Within a scheme of categorization of oscillatory reaction mechanisms, we identify oxygen as a type Y essential species and coIII a type Z essential species. Both NADH and the native HRP enzyme are nonessential species, likely of type C. With the results of the experiments presented combined with other known facts about the experimental system, the category of the oscillator is determined to be 1CW. The assignment of the roles of the species and the categorization of the oscillator allows for the deduction of parts of the reaction mechanism.