Demethylating methyl phenyl ethers is challenging, especially when the products are catechol derivatives prone to follow‐up reactions. For biocatalytic demethylation, monooxygenases have previously been described requiring molecular oxygen which may cause oxidative side reactions. Here we show that such compounds can be demethylated anaerobically by using cobalamin‐dependent methyltransferases exploiting thiols like ethyl 3‐mercaptopropionate as a methyl trap. Using just two equivalents of this reagent, a broad spectrum of substituted guaiacol derivatives were demethylated, with conversions mostly above 90 %. This strategy was used to prepare the highly valuable antioxidant hydroxytyrosol on a one‐gram scale in 97 % isolated yield. Efficient demethylation of methyl phenyl ethers was achieved by biocatalytic O2‐free methyl transfer to thiols forming thio ethers. Since the methyl group is not cleaved from the sulfur under these conditions, the thiol compound acts like a trap driving the reaction towards completion.