MitoP is a phenol product produced by the reaction of H2O2 with the ratiometric mass spectrometry probe MitoB . MitoB contains a triphenylphosphonium cation component that drives its accumulation in mitochondria where its arylboronic moiety selectively reacts with H2O2 to produce MitoP. Quantifying the MitoP/MitoB ratio by LC-MS/MS reflects the mitochondrial matrix H2O2 concentration.