The published microwave spectrum of azulene is re-analysed to yield the following values of the centrifugal distortion constants (units of kHz): Taaaa -1.45�0.76; Tbbbb -0.05�0.07; Taabb +0.70�0.42; Tabab -0.47�0.17. ��� The inertial defect Δ = Ic-Ia-Ib, typically positive in monocyclic compounds, is negative in azulene. According to the theory of Oka and Morino, the largest contribution to Δ is the vibrational part, which depends on the frequencies of the normal vibrations and on the Coriolis coupling constants. For the latter, a modified ?uniform coupling? approximation is proposed, based on sum rules applicable to planar molecules. Calculations are then carried out on pyridine, naphthalene, and azulene in which the change of sign of Δ, in two-ring, and presumably larger, aromatics is traced to the influence of low- frequency out-of-plane vibrations. For azulene, the values of Δ in a vibrationally excited state and an electronically excited state are also considered.