Electrical measurements are combined with the technique of secondary‐ion mass spectrometry (SIMS) in order to experimentally analyze and correlate the diffusion and activation of ion‐implanted selenium in GaAs. A theory is presented based on the assumption of four chemically different species of selenium: (1) substitutional selenium, (2) interstitial selenium, (3) selenium complexed with a gallium vacancy, and (4) precipitated selenium. It is proposed that the interaction between these four species dictates the resulting redistribution and electrical activation of ion‐implanted layers. The factors governing these interactions are investigated, and it is speculated that only substitutional selenium is a shallow donor. In addition, it is speculated that the species responsible for redistribution of impurity profiles is the selenium‐gallium vacancy complex. Precipitates and interstitial selenium appear to neither diffuse nor act like donors in GaAs. A model is developed which formalizes these observations in a set of five coupled differential equations. By employing a minimum number of simplifying assumptions, we are able to extract quantitative predictions from this model which accurately describe not only our experimental results but those of other workers.