The polymorphism of crystalline cyclooctanone and cyclononanone has been investigated using X-ray diffraction, nuclear magnetic resonance spectroscopy and differential thermal analysis. Both compounds undergo phase transitions below room temperature; the crystalline phases stable at room temperature are highly disordered and are isostructural with corresponding phases of several widely differing compounds, including cyclooctane, cycloheptatriene, β-fluorine and γ-oxygen.