A critical velocity ionization experiment was carried out with a heavily instrumented rocket launched from Wallops Island at dawn on May 13, 1986. Two neutral barium beams were created by explosive shaped charges released from the rocket and detonated at 48¿ to B at altitudes near 400 km and below the solar UV cutoff. Critical velocity ionization was expected to form a detectable ion jet along the release field line, but instead an ion cloud of fairly uniform intensity was observed stretching from the release field line across to where the neutral barium jet reached sunlight. The process creating these ions must have been present from the time of the release and the efficiency is estimated to be equivalent to an ionization time constant of 1800 s. This ionization is most likely from collisions between the neutral barium jet and the ambient atmospheric oxygen, and if so, the cross section for collisional ionization is 9¿10-18 cm2. A critical velocity ionization process may have been present during the first few tenths of a second after release, but its efficiency cannot have exceeded an equivalent ionization time constant of about 1800 s. ¿ American Geophysical Union 1990