Hypothesis: The viscosity and stability of CO2/water foams at elevated temperature can be increased significantly with highly viscoelastic aqueous lamellae. The slow thinning of these viscoelastic lamellae leads to greater foam stability upon slowing down Ostwald ripening and coalescence. In the aqueous phase, the viscoelasticity may be increased by increasing the surfactant tail length to form more entangled micelles even at high temperatures and salinity. Experiments: Systematic measurements of the steady state shear viscosity of aqueous solutions of the diamine surfactant (C16-18N(CH3)C3N(CH3)(2)) were conducted at varying surfactant concentrations and salinity to determine the parameters for formation of entangled wormlike micelles. The apparent viscosity and stability of CO2/water foams were compared for systems with viscoelastic entangled micellar aqueous phases relative to those with much less viscous spherical micelles. Findings: We demonstrated for the first time stable CO2/water foams at temperatures up to 120 degrees C and CO2 volumetric fractions up to 0.98 with a single diamine surfactant, C16-18N(CH3)C3N(CH3)(2). The foam stability was increased by increasing the packing parameter of the surfactant with a long tail and methyl substitution on the amine to form entangled viscoelastic wormlike micelles in the aqueous phase. The foam was more viscous and stable compared to foams with spherical micelles in the aqueous lamellae as seen with C12-14N(EO)(2) and C16-18N(EO)C3N(EO)(2). (C) 2018 Published by Elsevier Inc.