The momentum balance between all emerging particles (electron, recoil ion and projectile) was explored for single ionization of helium by 3.6 MeV/u Ni 24+ impact in a kinematically complete experiment. Technically this was achieved by integrating a novel 4π low-energy electron analyzer into a high-resolution cold-target recoil-ion momentum spectrometer. More than 90% of the “soft electrons” (0 eV ≤ E e ≲ 50 eV) are ejected in forward direction with a most probable longitudinal energy (along the ion-beam) of E e⊥ ≈ 3 eV . Not the projectile, but the backwards ejected recoil-ion ( E R⊥ ≈ 0.4 meV ) compensates the electron longitudinal momentum except of a small contribution from the inelasticity of the reaction. Energy losses of the 0.2 GeV projectiles as small as ΔE p E p = 3.4 × 10 −7 , transverse momentum balances, as well as electron energy and angular distributions for defined final recoil-ion charge state become accessible with this technique.