Stability and aggregation structures of various economically viable surfactants for CO2 are reported. The compounds are either commercially available octylphenol nonionics (Triton X-100, X-100 reduced, and X-45) or custom-made analogues of aerosol-OT (J. Am. Chem. Soc. 123 (2001) 988). These were selected to reveal the influence of chain terminal group structure, namely highly methylated t-butyl units, on solubility and aggregation in CO2. In addition the mean ethylene oxide block length is varied for the Triton surfactants (X-100 similar toEO(10), X-45 similar to EO8). High-pressure small-angle neutron scattering (SANS) experiments revealed the presence of aggregates, consistent with spheroidal reverse micelles. The nonionics show a temperature and pressure dependence on solubility. These results confirm the special affinity of highly methyl-branched tails for CO2. However, none of these systems were able to disperse significant amounts of water or brine; therefore hydrated reversed micelles or microemulsion droplets were not stabilized. Hence the utility of these cheap methyl-branched surfactants in CO2 is limited, and so groups of greater CO2-philicity are needed to achieve the goal of water-hydrocarbon surfactant-CO2 dispersions. (C) 2003 Elsevier Science (USA). All rights reserved.