Enthalpies of combustion and vaporization at 25.0 °C have been measured for some saturated, straight-chain oxa-compounds. Enthalpies of formation at 25.0 °C have been derived for the compounds in the liquid and gaseous states. The results are: ΔH f o(l)/kJ mol −1 ΔH f o(g)/kJ mol −1 3,5-Dioxaheptane −450.4 ± 0.8 −414.8 ± 0.8 3,5,7-Trioxanonane −625.8 ± 1.0 −581.1 ± 1.0 3,5,7,9-Tetraoxaundecane −794.6 ± 1.2 −741.0 ± 1.4 3,5,7,9,11-Pentaoxatridecane −968.4 ± 1.4 −905.9 ± 1.9 3,6-Dioxaoctane −451.4 ± 1.0 −408.2 ± 1.0 The compounds with next-nearest-neighbour oxygen atoms are shown to be stabilized by (16.9 ± 0.2) kJ mol −1 per OCO interaction compared with monoethers. The results are summarized in terms of an Allen bond energy scheme and the OCO interaction parameter is calculated. The Allen scheme fails in predicting the ΔH f o(g) value for 3,6-dioxaoctane, which has an “apparent strain” of 10 kJ mol −1. The enthalpy of isomerization for CH 2OCH 2CH 2O-(g) = CH 2OCH 2OCH 2-(g) has been calculated, and is in good agreement with that for 1,4-dioxane(g) = 1,3-dioxane(g). Strain in a number of six-membered, cyclic oxa-compounds is discussed in the light of the results of this investigation.