The standard (p(o) = 0.1 MPa) enthalpies of combustion at the temperature T = 298.15 K were measured by static-bomb calorimetry for crystalline 4-nitropyridine N-oxide and for pyridine-3-carboxylic acid N-oxide. The standard enthalpies of sublimation at T = 298.15 K were measured for 4-nitropyridine N-oxide by microcalorimetry and for pyridine-3-carboxylic acid N-oxide by a mass-loss effusion technique. -Delta(c)H(m)(o)(cr)/(kJ.mol(-1)) Delta(cr)(g)H(m)(o)(kJ.mol(-1)) 4-O2N-c-C-5 H4NO 2556.3 +/- 0.5 108.9 +/- 0.3 3-HO2C-c-C5H4NO 2698.2 +/- 0.7 152.3 +/- 1.9 From the standard molar enthalpies of formation of the gaseous compounds, the molar dissociation enthalpies of the (N-O) bonds were derived : D(N-O)/(kJ.mol(-1)) in 4-O2N-c-C5H4NO, 258.5 +/- 5.3; in 3-HO2C-c-C5H4NO, 252.8 +/- 2.7. The standard molar enthalpies of sublimation of 2-H3C-c-C5H4NO and 3-H3C-c-C5H4NO were measured by microcalorimetry : (78.2 +/- 2.2) kJ.mol(-1) and (82.2 +/- 2.4) kJ.mol(-1), respectively, permitting the comparison of D(N-O) values in several pyridine N-oxide derivatives.