In this research, we studied the effect of ethylene carbonate (EC) on thermodynamic, transport and optical properties such as electrical conductance, refractive indices and activity coefficients of aqueous 1-ethyl-3-methylimidazolium bromide, [EMIm]Br solutions at T = (298.2, 308.2 and 318.2) K and 0.1 MPa. Electrical conductance was measured for [EMIm]Br ionic liquid from 0.0029 to 0.2500 mol kg(-1) in different mass fractions of ethylene carbonate (EC) in water + EC mixtures (W-EC/W-mixture% = 10, 20 and 30). Limiting molar conductivity (Lambda(degrees)) and ion association constant (K-A) were calculated through Fuoss-Onsager equation and used to obtain the thermodynamic functions. Refractive index measurements were carried out for the binary and ternary water + [EmIm]Br + EC mixtures. The measured data were used to derive the refractive index changes of mixing (Delta n(D)) and the values were correlated by Redlich-Kister and the Cibulka equations for binary and ternary mixtures, respectively. Moreover, the results relating to the mean activity coefficient measurements for [EMIm]Br in the EC + water solvent mixtures were presented by potentiometric method. The potentiometric measurements were performed on the electrochemical cell of the type Br-ISE vertical bar [EMIm]Br (m), EC (wt.%), H2O (1-wt)%vertical bar [EMIm]-ISE, in various mass fractions of EC in water (0, 10, 20 and 30%) over total ionic strength from 0.0085 to 2.500 mol kg(-1). The experimental results were correlated by the Pitzer equation and the values of Pitzer ion-interaction parameters beta((0)), beta((1)) and C-phi were evaluated. Finally the parameters were utilized to calculate the values of the osmotic coefficients, excess Gibbs free energies and the solvent activity for the whole series of investigated system. (C) 2019 Elsevier B.V. All rights reserved.