The title amino acid esters (1a) (1c) and (1e) were prepared in high purity. The enthalpies of combustion Delta H-c(0)(1) of (1a-e) were measured using an isoperibolic calorimeter. The enthalpies of vaporization Delta H-vap of(1b-e) were obtained from the temperature function of the vapour pressure measured in a flow system. The following enthalpies of formation have been obtained in the liquid and the gas phase respectively : Delta H-f(0)(1) and Delta H-f(0)(g) (in kJ mol(-1)) for (1a)-468.61 +/- 0.84, -419.4 +/- 1.4; (1b) -472.7 +/- 1.0; (1c) -542.1 +/- 1.1, -488.1 +/- 1.2; (1d) -534.80 +/- 0.96 -479.9 +/- 1.1; (1e) -504.66 +/- 0.83. The values of the synergetic interaction of geminal substituents are discussed in relation to the branching at the central C-atom.