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Rate constants and efficiencies of the oxidative bleaching of triplet‐excited methylene blue by different arene diazonium salts are determined by flash photolysis and stationary experiments. In the same way the rate constants of methylene blue photoreductions by diazabicylo[2,2,2]‐octane (DABCO), sodium tetraphenyl borate (TPB), sodium ethylenediamine tetraacetate (EDTA), and allyl thiourea (ATU) are determined. According to these results TPB reduces methylene blue photochemically not exceedingly fast (ke = 1.5 × 107M−1s−1) but with a high efficiency (η = 0.8). At high diazonium salt concentrations (10−1 – 10−2 M), the oxidation and the reduction mechanisms of the triplet‐methylene‐blue sensitized dediazoniation of arene diazonium salts have similar efficiencies at high as well as at low light intensities. At low diazonium salt concentrations and with diazonium salts of low reactivity, the reduction mechanism is favoured over the oxidation mechanism.