Coprecipitation of active pharmaceutical ingredients (API) with additives can provide the ability to add functionality to API's at early stages of drug discovery and synthesis. It is with this objective that a number of drugs were recrystallized in the presence of structurally related additives. The rapid expansion of supercritical solution (RESS) process was evaluated to recrystallize the drug-additive mixtures. Results of RESS aided coprecipitation studies involving twelve drug-additive mixtures are reported in this manuscript. Characterization of these mixtures revealed a number of interesting phenomena. These include habit modification, solubility enhancement, particle size reduction. eutectic formation, reduction in crystallinity, amorphous conversion, hydrate formation, polymorph conversion and selective extraction. In viewing each of these phenomena from an application standpoint, this manuscript serves as proof of concept for altering the physicochemical and mechanical properties of API's using supercritical fluid (SCF) coprecipitation. It was concluded from these studies that the rapid crystallization conditions offered by the SCF media alone does not provide the ability to consistently dope crystals. Competing mechanisms based on the relative solubility of drugs and additives in SCF media, as well as the selectivity of SCF solvents are to be taken into consideration while undertaking coprecipitations. (C) 2008 Elsevier B.V. All rights reserved.