Intense energetic beams of hydrogen cluster ions characterized by an atom flux density of several A0/cm2, a kinetic energy per atom of several eV, and a number of atoms per charge in the cluster ions of about 105are markedly influenced by collisions of the cluster ions among themselves and with background gas particles. This is demonstrated by acceleration of a partially ionized cluster beam with high voltages in the range of 75–350 kV and measurement of the attenuation of the neutral component. The attenuation varies exponentially with the ionizing electron current; the attenuation constantbis proportional toU1.7a, whereUais the acceleration voltage. Straightforward extrapolation of  bto the conditions prevailing in fully ionized cluster beams leads to attenuations down to about 20% which is not too far from the measured value scattering around 40%.