The quantification of peroxynitrite (ONOO⁻, PON) and hydrogen peroxide (H₂O₂) is intrinsically difficult as both species show similar oxidative features located within a narrow potential. The sub-second lifetime of ONOO⁻ at neutral pH further complicates the analysis. In this paper, we examine the electrocatalytic activity of cobalt phthalocyanine tetracarboxylic acid (CoPc–COOH) loaded reduced graphene oxide (rGO) films towards peroxynitrite and hydrogen peroxide detection. The rGO/CoPc–COOH matrix is synthesized by the reaction of graphene oxide (GO) and CoPc–COOH at 90 °C for 5 h under ultrasonication. The integration of CoPc–COOH and the reduction of GO to rGO was confirmed by X-ray photoelectron spectroscopy, FTIR, Raman, UV-vis spectroscopy and electrochemistry. The rGO/CoPc–COOH film showed high electrocatalytic activity and specificity for ONOO⁻ at anodic potential with a sensitivity of ≈11.5 ± 1 nA nM⁻¹ and a peroxynitrite detection limit of ≈1.7 nM. The rGO/CoPc–COOH films further exhibited electrocatalytic reduction of H₂O₂ with a sensitivity of 14.5 μA mM⁻¹ and a detection limit of ≈60 μM for H₂O₂.