We have used a Sodar (acoustic doppler sounder) to characterize the behavior of volcanoes, and in particular, to measure the vertical velocities of volcanic jets. We have analyzed more than 100 explosions on Stromboli volcano (Eolian islands). First results show vertical speeds close to the vent ranging from 20 m/s to 80 m/s. The uncertainty in the measurements is close to 10%, due mainly to the inaccurate knowledge of the sound speed in volcanic jets. This is small, compared to other volcanological methods. This method is well suited for monitoring temporal variations during an eruption and should be particularly interesting for the study of the eruptive dynamics of lava fountains. The Doppler spectrum in explosions appears to be related to different types of echoes and contains information on particle sizes in the jet. ¿American Geophysical Union 1992