Co(salen) [cobaltous bis(salicylaldehyde)ethylenediamine] complexes are well-known O-2 carriers in solution. In the solid phase, these complexes exhibit some O-2 binding but detailed studies have been complicated because few of the known polymorphs of Co(salen) actually bind O-2. The O-2 binding results for nanoparticulate Co(salen) are presented in this study. Rod-shaped Co(salen) nanoparticles, roughly 100 nm in diameter, were recrystallized by spraying a methylene chloride solution of commercially obtained Co(salen) into supercritical carbon dioxide. Temperature-programmed desorption, thermogravimetric analysis, and a Rubotherm magnetic suspension balance measurements reveal a reversible O-2 uptake of similar to 1.51 mmoll/(g nanoparticles) at 25 degrees C, consistent with a binding stoichiometry involving a bridging peroxo unit between two Co centers. In contrast, no measurable O-2 uptake was observed with the commercial Co(salen). These results clearly show the potential for bottom-up design of nanoparticulate metal complexes for enhanced O-2 storage and other applications. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1040-1045, 2009