Gas-phase standard enthalpies of formation of selected ureas and s-triazines were calculated at the CCSD (F12*)(T)/cc-pVTZ-F12//omega B97X-D/cc-pVTZ level of theory by employing isodesmic reactions. The following values were obtained (T = 298.15 K, units: kJ mol (1), estimated confidence interval 95%): urea, -231.9 +/- 2.8; biuret, -430.0 +/- 4.5; triuret, -620.3 +/- 6.3; cyanuric acid, -451.6 +/- 6.3; ammelide, -303.9 +/- 6.4; ammeline, -106.5 +/- 6.2; melamine, 70.1 +/- 7.0. The standard enthalpies of formation of methanimine and methylamine, which were required for the isodesmic reactions, were calculated from atomization reactions by using several variants of the HEAT approach. The following results were considered most reliable (T = 298.15 K, units: kJ mol (1), estimated confidence interval 95%): methanimine, 89.0 +/- 1.0; methylamine, -20.7 +/- 1.0. (C) 2017 Elsevier B.V. All rights reserved.