A one‐dimensional kinetic code is used to study the effect of ion‐neutral (charge exchange and elastic scattering) and ion‐ion collisions on plasma flow in the downstream region of an electron‐cyclotron‐resonance plasma etching system. Ions are assumed to leave the source region at the Bohm velocity. Argon, nitrogen, and CF4plasmas are simulated, assuming that the dominant ion species are Ar+, N2+, and CF3+, respectively. Results show that charge exchange and elastic scattering collisions play a significant role in reducing the electrostatic potential variation in the downstream region. For neutral gas pressures above ∼1 mTorr, the potential drop in the downstream region is small, which means that most of the energy with which ions hit the substrate surface is gained while crossing the substrate sheath region. Although the effect of ion‐ion collisions on the plasma potential profile and on the ion distribution function is weak, ion‐ion collisions are responsible for transferring energy from the parallel to the perpendicular direction and hence increasing the perpendicular ion temperature.