The molecular rearrangement of 5-methyl-4-nitrobenzofuroxan to 7-methyl-4-nitrobenzofuroxan was studied by means of ab initio and density functional theory. Experimentally obtained IR spectra and X-ray data support the applicability of the theoretical methods and allow for a complete assignment of the vibrational modes. The influence of the methyl substituent on the underlying tautomeric reaction was investigated in detail. Trends for the reactivity of 4-nitrobenzofuroxans with substituents in the 5-position were established on the basis of an energy partitioning, providing insight into the driving forces of the Boulton-Katritzky rearrangement. Rate constants were calculated for this reaction using different implementations of variational transition-state theory. Ln addition a reaction yielding 6-methyl-7-nitrobenzofuroxan was investigated as an alternative to the rearrangement: considered. An accurate treatment of relative energies of this competing reaction requires very demanding computational methods which could not be applied here. Therefore, energy corrections were estimated from smaller benchmark systems.