The thermolysis of ortho-nitrophenyl azide, yielding benzofuroxan and nitrogen, has been investigated using quantum chemical methods up to the CCSD(T)/6-311G(2d,p) level. The calculated activation barriers are in excellent agreement with experimental data. A comparison of the rate constants computed from various implementations of variational transition state theory with experimental data clearly identifies the computed reaction mechanism as the experimentally observed one. Neighboring-group assistance within the pyrolysis leads to a one-step mechanism, thus turning down intermediates as discussed in the literature.