The mechanism of the copper-containing enzyme quercetin 2,3-dioxygenase has been studied using hybrid density functional theory. This enzyme cleaves the O-heterocycle of a flavonol using dioxygen and releases carbon monoxide. Two different pathways for the dioxygen attack on the copper complex have been investigated, and the one where the first attack is on copper is found to be the energetically preferred one. By using this pathway the problem of having to go through a spin-orbit-induced spin crossing is also avoided. The adduct has three unpaired spins and is ideally suited for forming a dioxygen bridging structure, which occurs in the next step. Rather than cleaving the O-O bond in the next step, another C-O bond between dioxygen and the substrate is first formed. Finally, the O-O bond is cleaved, and CO is released in one concerted transition state with a very low barrier. The results are in good agreement with experimental findings. The mechanism is compared to the ones for other similar enzymes studied recently by similar methods.