We estimated the characteristic frequency of vibration of two dams located in southern Italy using spectral amplitudes of 13 local earthquakes recorded by three-component digital stations installed on top of the structures and on the free field. One of the dams (ARV), located in Arvo lake, is an earth dam, and the other (TRP) is an arch dam located in the lake Ampollino. We separated the source and path effects from the records on the dams by using spectral ratios between the horizontal and the vertical components of motion (H/V ratios). We also calculated simultaneous inversions for source, site, and path effects using different site constraints. In general, the characteristic frequencies (fn) retrived from both approaches are consistent with the expected natural frequency of resonance of the structures. For ARV, we estimated from the spectral records an fn = 2.5 Hz and a value of 2.0 Hz when we calculated the one-dimensional response using the SHAKE code (Schnabel et al., 1972). For TRP, the results of the inversion and the spectral ratio between the free-field site give fn = 8.0 Hz. We concluded that the inversion technique gives reliable estimates of fn for the sites analyzed.