Icons play an important role in modern interfaces and therefore recent empirical research has focused on enhancing icon processing — that is, icon perception and icon function understanding. However, in existing sets, icons vary simultaneously across different icon characteristics, confusing the contribution of each to icon processing. We developed icon design principles for aesthetics, complexity, and concreteness, and used them to create 64 icons that varied independently along each characteristic. Participants reported the icon function and rated each icon in terms of aesthetics, complexity and concreteness. The manipulated characteristics had independent effects on icon processing, with two exceptions, for which we propose evidence-based solutions. Based on these findings we propose guidelines for designing icons for research purposes.