A13C nuclear magnetic resonance spectroscopy study of 1-oxaspiro[5.5]undecanes1,4, and5as a function of temperature is reported. Compound1exists as a rapidly equilibrating mixture of a major and a minor conformer (1aand1b) at room temperature that can be observed at low temperature. By comparison, 1,7-dioxaspiro[5.5]undecane2is conformationally rigid in a single conformation (2a) at room temperature. This completely different behavior demonstrates the importance of theendoand theexoanomeric effects in the acetal function.Keywords: spiro ethers, conformational analysis,13C NMR.