The role of energetic ion bombardment in causing directionality in plasma-assisted etching is well established. However, the detailed mechanisms by which ion bombardment accelerates etching reactions are not well understood. Experiments have been carried out using controlled beams of particles representative of a plasma-assisted etching environment and the results of these experiments can be qualitatively related to plasma-assisted etching phenomena. The results of these controlled beam experiments also provide some insight concerning the basic mechanisms responsible for ion-assisted etching. Evidence will be summarized which indicates that, for the silicon-halogen system, the primary role of energetic ion bombardment is to accelerate the formation of a volatile product from a partially reacted surface layer. This conclusion cannot be extended in general to other gas-solid combinations and some data obtained from metal-halogen systems will be noted. © 1988 IOP Publishing Ltd.