The feasibility of various metal chelates as components of acellular resuscitation fluids is examined, with particular emphasis on the bonding mode and kinetics of the dioxygen adduct formed. End-on (Pauling type) or bridged peroxo-dimer structures using iron(II) or cobalt(II) as the active metal ion are currently the most promising candidates for investigation. Criteria which must be met in order to confer desirable O2-binding properties upon the coordinated metal ions are described and ligand systems (porphyrins, Schiff-bases, lacunar macrobicycles) which help fulfill these criteria are critiqued. An important aspect of acellular O2-delivering systems is the mode of dispersion, which in turn is a function of the properties of the metal chelate; this question is broached, for general application, in terms of true solutions, emulsions and colloidally suspended solids.