We investigated the radial implosion of a deuterium plasma in a theta pinch in which the initial level of ionization was controlled. Measurements of the diamagnetism of the plasma during the implosion were related to the mass of gas in motion. The latter depends on the interaction between the ions and the neutral atoms. The effective cross-section for momentum transfer from ions to neutrals is 4 × 10−15 cm2, close to the charge exchange cross-section for protons and hydrogen atoms.