Aggregation behaviour and thermodynamics of mixed micellization of two cationic gemini surfactants, 1,3-bis(dodecyl-N,N-dimethylammonium bromide)-2-propanol [12-3(OH)-12,2Br-] and 1,4-bis(dodecyl-N,N- dimethylammonium bromide)-2,3-butanediol [12-4(OH)2-12,2Br -] with a room temperature ionic liquid (RTIL), 1-dodecyl-3- methylimidazolium bromide (DMIL) in water and in binary mixed media of water and organic solvents [1,4-dioxane (DO) and ethylene glycol (EG)] have been investigated by means of conductivity, fluorescence and dynamic light scattering measurements at T = 303.15 K. The chemical nature of the spacer groups of gemini surfactants and the organic co-solvents have significant effect on the aggregation behaviour and the thermodynamics of mixed micellization. The mixed micellization behaviour with possible interactions between DMIL and 12-3(OH)-12,2Br-/12-4(OH)2-12,2Br- have been demonstrated. The process of micellization of DMIL + 12-4(OH) 2-12,2Br- is comparatively delayed than that of DMIL + 12-3(OH)-12,2Br- in a given solvent system. The micellization process is progressively delayed with increasing bulk mole fraction of DMIL (αDMIL). The hydrodynamic diameter of DMIL + 12-4(OH) 2-12,2Br- micelle is smaller than that of DMIL + 12-3(OH)-12,2Br- micelle. The micellar mole fraction of DMIL estimated using different theories (Rubingh, Rodenas and Motomura) is higher than that expected for an ideal system. The attractive interactions among the mixed micellar components are described on the basis of Rubingh's β parameter, activity coefficient values, experimental and ideal cmc values, and the values of micellar mole fractions. The negative values of excess Gibbs energy (GE) in aqueous and mixed solvent media indicate that the non-ideal mixed micellar systems are more stable than the corresponding ideal systems. A micelle structure becomes more open when the dielectric constant of the solvent is reduced. © 2013 Elsevier Ltd. All rights reserved.