The standard (p o = 0.1 MPa) enthalpies of formation Δ f H m o (cr) at the temperatureT= 298.15 K were determined using static-bomb calorimetry for crystalline 5-methoxybenzofurazan (5MeOBFN), 5-methoxybenzofurazan-1-oxide (5MeOBFNO), 5-methylbenzofurazan-1-oxide (5MeBFNO), and 4-nitrobenzofurazan-1-oxide (4NO 2 BFNO); using rotating-bomb calorimetry for 5-chlorobenzofurazan-1-oxide (5ClBFNO). The standard enthalpies of sublimation Δ c r g H m o atT= 298.15 K were measured by microcalorimetry and, from the enthalpies of formation of the gaseous compounds, the dissociation enthalpiesD m o of the (N + -O - ) dative covalent bonds were derived.Δ f H m o (cr)/(kJ.mol - 1 ) Δ c r g H m o /(kJ.mol - 1 )D m o (N-O)/(kJ.mol - 1 ) 5MeOBFN 57.9+/-1.7 89.2+/-0.7 5MeOBFNO 52.3+/-3.3 96.0+/-1.6 248.0+/-3.7 5MeBFNO 175.2+/-1.8 92.2+/-1.2 249.8+/-3.2 5ClBFNO 193.5+/-1.8 81.2+/-1.8 244.1+/-3.5 4NO 2 BFNO 202.6+/-2.1 97.3+/-1.6 255.9+/-6.7The weighted mean value ofD m o (N-O)/(kJ.mol - 1 ) in benzofurazan-1-oxide derivatives has been assessed as (249.9+/-0.6)/(kJ.mol - 1 ).