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The ammonia monooxygenase (AMO) is a key enzyme in ammonia oxidising archaea, which are abundant and ubiquitous in soil environments. The AMO belongs to the copper-containing membrane monooxygenase (CuMMO) enzyme superfamily, which also contains particulate methane monooxygenase (pMMO). Enzymes in the CuMMO superfamily are promiscuous, which results in co-oxidation of alternative substrates. The phylogenetic and structural similarity between the pMMO and the archaeal AMO is well-established, but there is surprisingly little information on the influence of methane and methanol on the archaeal AMO and terrestrial nitrification. The aim of this study was to examine the effects of methane and methanol on the soil ammonia oxidising archaeon 'Ca. Nitrosocosmicus franklandus C13'. We demonstrate that both methane and methanol are competitive inhibitors of the archaeal AMO. The inhibition constants (K