Anomalous carrier profiles in BF+2‐ion‐implanted silicon are investigated by theC‐Vand incremental‐sheet‐resistivity methods. An anomalous tail extends about twice as deep as the normal profile, while the peak carrier concentration is around 1/100 that of the main peak. The origin of this anomalous tail is investigated by comparing the profiles of B+, B++F+, and BF+2ions implanted using postacceleration‐ and preacceleration‐type implanters. High‐energy B+ions generated between a mass‐separation magnet and a postacceleration tube are found to be responsible for the observed profiles.