The standard (p degrees = 0.1 MPa) molar enthalpies of combustion at the temperature T = 298.15K were measured by static-bomb calorimetry for crystalline 2,2’6,6’-tetraethylazobenzene N,N-dioxide, {2,6-(C2H5)(2)C6H3N(O).}2; 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene, 2,4,6-{CH3C(CH3)(2)}3C6H2NO; and 2,4,6-tri(1,1-dimethylethyl)nitrobenzene, 2,4,6-{CH3C(CH3)(2)}3C6H2NO2. The standard molar enthalpies of sublimation at T = 298.15 K of the tri(1,1-dimethylethyl) compounds were measured by microcalorimetry. [GRAPHICS]. The standard molar enthalpy of decomposition of {2,6-(C2H5)(2)C6H3N(O).}2 to form 2,6-(C2H5)(2)C6H3NO(g) at T = 298.15K was measured by microcalorimetry : Delta(dec)H(m) degrees/(kJ . mol(-1)) = (200.6 +/- 5.6). Application of group-additivity schemes applied to nitrosobenzene and nitrobenzene derivatives shows that 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene is unstrained whereas the corresponding nitrocompound shows considerable strain in accord with an X-ray structure analysis demonstrating that steric hindrance prevents dimerization of the nitrosoderivative.