Ab initio molecular orbital theory using the 3-21G( ∗), 6-31G ∗, 6-31G ∗∗, 6-311G ∗∗ basis sets, Møller-Plesset perturbation theory [MP2/3-21G( ∗)//3-21G( ∗), MP2/6-31G ∗//6-31G ∗, MP2/6-31G ∗∗//6-31G ∗∗, MP2/6-311G ∗∗//6-311G ∗∗], and density functional theory (pBP/DN ∗∗) were used to calculate the geometries and energies of dihydrodioxins (3,4-dihydro-1,2-dioxin, 3,6-dihydro-1,2-dioxin, 4 H-1,3-dioxin (1,3-diox-4-ene), and 2,3-dihydro-1,4-dioxin (1,4-dioxene). Frequency calculations show that the four dihydrodioxins exist in the half-chair conformation and that their boat conformers are transition states. Hyperconjugative orbital interactions (stereoelectronic effects) including n O(3) → σ ∗ C(2)– O(1) , n O(3) →σ ∗ C(2)– H ax , are observed in 4 H-1,3-dioxin. Ab initio (3-21G( ∗), 6-31G ∗) and MP2/6-31G ∗//6-31G ∗ calculations were used to obtain geometry optimized structures and conformational energies (−Δ G° or “ A values”, kcal/mol) of the rotamers in the half-chair conformers of 2-alkyl-4 H-1,3-dioxins and 2-trimethylsilyl-4 H-1,3-dioxin [MP2/6-31G ∗//6-31G ∗: CH 3(2.95), C 2H 5(2.89), iso-C 3H 7(2.97), tert-C 4H 9(7.34), neo-C 5H 11(2.16), Si(CH 3) 3(4.45)]. The calculated conformational energies of the 2-alkyl-4 H-1,3-dioxins and 2-trimethylsilyl-4 H-1,3-dioxin are larger than those calculated for the corresponding alkylcyclohexanes and 4-alkylcyclohexenes. Plots of the calculated conformational energies of the 2-alkyl-4 H-1,3-dioxins and 2-trimethylsilyl-4 H-1,3-dioxin versus the calculated −Δ G° values of the correspondingly substituted cyclohexanes (slope=1.320 and r=0.989) and 4-substituted cyclohexenes (slope=2.150 and r=0.968) are linear. Hyperconjugative orbital interactions are also observed in the 2-alkyl-4 H-1,3-dioxins. The C(2)–H ax bond lengths are longer than the C(2)–H eq bond lengths and the C(2)–O(1) bond lengths are longer than the C(2)–O(3) bond lengths in 4 H-1,3-dioxin and in the 2-substituted 4 H-1,3-dioxins. The C(4)–O(3) bond lengths in the 4 H-1,3-dioxins are generally 1.407 or 1.408 Å. In the 2-substituted 4 H-1,3-dioxins, the O(1)–C(2)–O(1) bond angles vary from 110.4 to 112.8° and the C(2)–O(3)–C(4) and C(7)–C(2)–O(3) bond angles in the most stable axial conformer are larger that the corresponding angles in its most stable equatorial conformer.