A series of conformationally biased 2-, 4-, and 5-alkyl-l,3-dioxanes has been studied through acid-catalyzed equilibration of diastereoisomers and by nuclear magnetic resonance spectroscopy. The preferences of the alkyl substituents for the equatorial (over the axial) position are substantially different from corresponding preferences in cyclohexane and depend on the location in the dioxane ring. Quantitatively, these preferences (in kilocalories/mole) are Me-2, ^3.55; 2.9; 0.8; 0.7; 1.0; 1.0; 1.4. The data are rationalized in terms of the shape of the 1,3-dioxane ring and the "size" of the free electron pair on oxygen.