Аннотация:An experimental investigation is performed of the dynamics of attachment domains in a beam-driven gas discharge in O2+He mixtures. It is shown that the velocity with which domains move decreases as a function of the discharge current and as the O2 concentration in the mixture decreases. In particular, it can be less than the speed of sound. The observed behavior is explained by means of a linear approach based on numerical solution of the electron Boltzmann equation. The primary mechanism for propagation of disturbances resulting from the attachment instability of the plasma is drift of negative ions.