The current work investigates the effect of ethylene glycol (EG) on the surface and aggregation behavior of two ester-containing gemini surfactants (i.e., esterquat and betainate). EG, as a cosolvent, was added to aqueous surfactant solutions in three different percentages including 20, 35, and 50 (v/v %). The formation of surfactant aggregates was investigated using dynamic light scattering, zeta potential, and transmission electron microscopy. Several physical-chemical properties such as critical micelle concentration (cmc), Gibbs free energy of micellization (Î"Gmic°), interfacial properties, degree of counterion dissociation (α), and morphology of aggregates were also determined in the absence and presence of EG. By increasing the concentration of EG in the surfactant solutions, the cmc and the minimum area per surfactant molecule (Amin) increased. Moreover, smaller aggregates were formed in the solutions with lower dielectric constant because of the increasing head-head repulsions. The ellipsoidal aggregates in water transformed into a spherical aggregate in the EG + water system. The zeta potential of the surfactant solutions decreased at higher EG concentrations, which was attributed to the formation of micelles with smaller hydrodynamic radii.