This paper focuses on the use of free-free resonance testing (FFR testing) applied to the characterization of stabilized silt and sand specimens treated in situ by deep soil mixing. The aim of FFR testing is to measure the natural frequencies of free vibration of the tested specimen. Compression and shear wave velocities, seismic moduli and Poisson's ratio can be determined from these frequencies. Block samples were taken from soil-cement columns installed at a test site near Paris, France. Specimens were cored from these blocks and submitted to FFR testing in the laboratory. It was found that both P-wave and S-wave velocities increase non linearly with unconfined compressive strength. A linear correlation between strength and dynamic stiffness was observed. Free-free resonance test results can be correlated with index parameters such as density and porosity and used for preliminary assessments of static stiffness as all these parameters varied linearly with measured wave velocities.