  Using the cold target recoil ion momentum spectroscopy (COLTRIMS) technique,
we have measured the simultaneous projectile and target ionization in
collisions of He$^+$ projectiles with a mixture of gaseous H$_2$ and D$_2$ for
an incident projectile energy of 650 keV. Motivated by Cooper et al. [Phys.
Rev. Lett. 100, 043204 (2008)], we look for differences in the ionization cross
section of the two isotopes with the highest resolution and statistical
significance. Contributions of the electron-electron and electron-nucleus
interactions have been clearly separated kinematically by measuring the
longitudinal and transverse momentum of the recoiling ion. We find no
significant isotope effect in any of our momentum distributions.
