The study of plasma etching mechanisms is made difficult by the presence of the plasma at the etching surface. However, etching in a plasma beam extracted from such a plasma allows: (1) mass analysis of beam species and of directly desorbing etching product, (2) simultaneous XPS analysis of the etching surface, and (3) ion acceleration or retardation without changing neutral species composition. The present beam was extracted from the unpowered electrode of a source operating under typical plasma etching conditions of 50 Pa and 27 MHz. Appearance potential mass spectroscopy was used to distinguish spectrometer fragmentation products from plasma radicals. Cl2 plasma beam reaction with undoped Si and with oxidized Al occurs only with ion bombardment, whereas with clean Al it occurs without ions, leading to isotropic (undercut) etching. Surface oxygen depletion and the onset of Al2Cl6 desorption were simultaneously monitored during the ’’initiation’’ phase of Al etching. Anisotropic Al etching in chlorocarbon plasmas is believed to be dependent on sidewall etching inhibition by chlorocarbon deposits. Mass analysis of beams from Cl2 plasmas containing 20% CCl4, CHCl3, or CH3Cl showed a similar product distribution of CmCln species in all three cases and, in the latter two cases, almost complete abstraction of H by Cl atoms to form HCl. The HCl neither enhances nor inhibits the reaction of Al with Cl2 plasma, nor does it react with clean Al in the absence of ion bombardment.