We tested the hypothesis involved in determining the dynamic response of soils using spectral ratios between horizontal and vertical components of ground motion. This hypothesis proposes that the vertical component of motion contains mainly the source effect and is approximately insensitive to site amplifications. We used the vertical component of strong-motion records from earthquakes of the region of Friuli, Italy, to evaluate the site response for different geologic conditions. S-wave source functions of the events used were first retrieved from the acceleration records using a generalized inversion in the frequency domain; then, an average site response was calculated for each site analyzed. The resulting site functions show small vertical amplifications for most of the sites (within a factor of 2). This result confirms the validity of the method to estimate S-wave site responses. However, when the S-wave arrivals are contaminated by surface waves, the vertical amplification may exceed the uncertainty expected by the spectral ratio technique.