High-purity samples of the title compounds - alkylsubstituted alpha-alkoxynitriles 1a-e and alpha-cyanocarboxylic acid esters 2a-c - were prepared, and the enthalpies of combustion Delta H-c(Theta)(1) of 1 and 2 were measured using an isoperibolic calorimeter. The enthalpies of vaporization Delta H-vap of 1 and 2 were obtained from the temperature function of the vapour pressure measured in a flow system. The following enthalpies of formation were obtained in the liquid phase and the gas phase, respectively : Delta H-f(Theta)(1) and Delta H-f(Theta)(g) (in kJ mol(-1)) for 1a -77.36 +/- 0.21, -35.65 +/- 0.66; 1b -162.80 +/- 1.59, -125.39 +/- 1.80; 1c -217.86 +/- 2.97, -169.28 +/- 3.01; 1d -223.50 +/- 2.90, -164.70 +/- 3.08; 1e -115.98 +/- 0.50, -69.50 +/- 0.56; 2a -309.53 +/- 0.54, -243.30 +/- 1.05; 2b -370.54 +/- 0.54, -311.96 +/- 0.63; 2c -507.69 +/- 1.51, -422.67 +/- 1.76. The values of the excess destabilization or stabilization energy caused by the geminal substituents are discussed in relation to the branching at the central C atom.