Four 3,4-dihydro-3-ortho-alkylphenyl-2H-1,3-benzoxazines have been synthesized. Enthalpies and entropies of activation for rotation about the phenyl–nitrogen bond have been calculated from the temperature dependence of the proton resonance spectra for the o-isopropyl (ΔS‡ = −4 e.u., ΔH‡ = 10.8 kcal/mole) and o-t-butyl (ΔS‡ = 0.4 e.u., ΔH‡ = 16.9 kcal/mole) derivatives. The barrier to rotation for the o-methyl and 2,6-dimethyl derivatives was too low to be measured by this procedure.