Experimental observations are reported on the range of compositions and geometries for which void-free "superfilling" of rectangular trenches occurs during copper electrodeposition from acid sulfate solutions containing poly(ethylene glycol), chloride, bis(sodium sulfopropyl)disulfide, and 1-(2-hydroxyethyl)-2-imidazolidine-thione. Observations were compared with predictions made with a model that included 17 species and reaction intermediates that participated in a network of 15 surface reactions as well as surface diffusion, and three homogeneous reactions. The numerical algorithm was designed to handle stiffness arising from reaction terms and computational efficiency required for interface movement. Accurately estimated values of the most sensitive model parameters were used to carry out simulations of shape evolution. One assumption was needed to bring the model into agreement with experimental trends for the "window" of superfilling conditions over the range investigated experimentally. Numerical results were found to track behavior typically observed during superfilling, such as an incubation period, bottom-up acceleration, and bump formation. Predictions of the distribution of adsorbed species during trench infill support the interpretation that superfilling arises from generation of a catalytic accelerator species within the trench in the presence of transport-limited competitive adsorption of suppressor and inhibitor additives. (C) 2008 The Electrochemical Society.