•Introduced double bond to 1-alkyl-3-(3-butenyl)imidazolium Br in water interactions.•Structure making tendency due to increasing B-coefficient with alkyl elongation.•Double bond in 1-butenyl chain leads to stronger interactions with water.•Positive Δμ1o≠ and Δμ2o≠ implies solute–solvent interactions and structure making effect. In the present study, volumetric and viscosimetric properties of aqueous solutions of imidazolium-based ionic liquids (ILs) with unsaturated and variety of different alkyl chain lengths, 1-alkyl-3-(3-butenyl)imidazolium bromide ((RC3=CimBr), R = methyl (C1), ethyl (C2) or butyl (C4)) were investigated. The experimental density and viscosity are measured as a function of the IL concentration at T = (293.15 to 313.15) K and atmospheric pressure (p = 101325 Pa). The volumetric property analysis is done to adjudge the effect of the introduced double bond, length of the alkyl chain on the imidazolium ring and hydrophilic non-hydrolyzable anion environment on the overall volumetric changes at infinite dilution. The experimental viscosity data are analyzed using the Jones-Dole equation to evaluate the viscosity B-coefficients. The results are discussed in terms of the interactions occurring in solution and compared with that available in the literature for corresponding saturated analogues.