Absolute cross sections of the associative ionization in the collision of He + with H − or D − have been measured in a merged beam experiment over a relative velocity interval ranging from 0.4 to 5 × 10 6 cm s −1 . At a given relative velocity, the two cross sections significantly differ at low velocities where the cross section for HeD + formation exceeds that of HeH + by as much as 35%. The difference, however, decreases with the velocity and falls in the experimental error above 2 × 10 6 cm s −1 . In both cases the energy dependence becomes compatible at low energy with an E −1 law.