A dense argon plasma(n e = 4 × 1018cm − 3, KT = 4 eV)is created and allowed to expand either into a vacuum or into a low density, partially ionized plasma. Expansion into a vacuum is shown to be dominated by electric‐field effects which accelerate ions to much higher energies(E ≲ 180 eV)than the original thermal energy. Expansion into a low density(n e = 1 × 1013cm − 3)10% ionized plasma of the same species was dominated by resonant charge exchange giving the appearance of momentum transfer. Expansion into a hydrogen plasma shows essentially complete momentum transfer as in a blast wave. Expansion of a hydrogen plasma into an ambient argon plasma shows spatially separated regions in which the hydrogen appears to collisionally snowplow the argon plasma.