The key physical processes governing resolution of gas-assisted focused electron-beam and ion-beam induced deposition and etching are analyzed via an adsorption rate model. The authors quantify how the balance of molecule depletion and replenishment determines the resolution inside the locally irradiated area. Scaling laws are derived relating the resolution of the deposits to molecule dissociation, surface diffusion, adsorption, and desorption. Supporting results from deposition experiments with a copper metalorganic precursor gas on a silicon substrate are presented and discussed. (c) 2007 American Vacuum Society.