Abstract The conformational behaviour of the halomethyl group in 5-position in 1, 3- dioxanes is determined quantitatively by low temperature nuclear magnetic resonance, and the origin of the axial preference of ca. 1 kcal. mole− 1 relative to the 5-methyl group is disclosed and discussed in terms of a local electrostatic attraction in XC-CC (H) Y fragments, having a double gauche conformation (X, Y electronegative elements).