Aluminum, magnesium, and lithium‐magnesium pellets were accelerated to velocities as high as 4.5 km/sec by means of an explosive. The ionization in the trails of these pellets was determined by measuring the reflection of a microwave signal from the trail. It was found that the ion density in the trail is an increasing function of the velocity and for the velocity range between 1 and 4.5 km/sec lies between 1010and 1015electrons per cm of path. It was observed that aluminum pellets do not produce ionization at velocities below approximately 2.9 km/sec, while magnesium leaves an ionized trail at velocities down to approximately 1.6 km/sec and lithium‐magnesium alloy down to approximately 1.3 km/sec.