Density matrix theory has been applied to obtain rate parameters for the interconversion between [1,2,5]thiadiazolo[3,4- e]benzofurazan-3-oxide and [1,2,5]thiadiazolo[3,4- e]benzofurazan-l-oxide (I), and for that between 7-methyl[1,2,3]triazolo[4,5- e]benzofurazan-3-oxide and 7-methyl[1,2,3]triazolo[4,5-ϵ]benzo-furazan-l-oxide (II). These systems show a superposition of two spectra of the AB type when exchange is slow; a single average AB spectrum is obtained under conditions of fast exchange. The activation parameters for the interconversions are ΔH ‡ = 92± 2 kJ mol −1, ΔS‡ = 13±5 J mol −1°C −1 for system (I), and ΔH ‡ = 91 ± 1 kJ mol −1, ΔS‡ = 19 ± 3 J mo −1 ΔC t-1 for system (II).