Abstract The conductances of tetradecyltrimethylammonium bromide (TTAB) + dodecyltrimethylammonium bromide (DTAB) mixtures over the entire mole fraction range of TTAB (αTTAB) were measured in pure water as well as in the presence of various aqueous ethylene glycol oligomers containing 5 and 10 wt% of each additive in their respective binary mixtures at 30 °C. From the conductivity data, the critical micellar concentrations (cmc) and degrees of micelle ionization (χ) for TTAB + DTAB mixtures were computed. From the slopes of the conductivity curves, the equivalent conductivities of the monomeric (Λm), monomeric in the presence of additive (Λm,a) and the micelle (Λmic) state were calculated, the results are discussed with respect to the glycol-additive effects in the whole mole fraction range of the binary mixture. The micellar parameters of both surfactants and of their mixtures show a significant dependence on the amount in comparison to the number of repeating units of glycol oligomers. These results have been explained on the basis of the medium effects of aqueous additive, which are drastically influenced by the increase in the amount of additive rather than by the increase in the number of repeating units. The non-ideality in TTAB + DTAB mixtures was evaluated by using the regular solution theory and Motomura’s formulation based on the excess thermodynamic quantities. It has been found that the regular solution interaction parameter (β) and micellar mole fraction () remain almost unaffected even in the presence of up to 10 wt% of each additive and molecular weight 4000. These results suggest that there are no significant interactions between the glycol oligomers and the micelles of single and mixed surfactants.