Densities, ρ, viscosities, η, and enthalpies of mixing of binary [P4 4 4 4][CnCOO]–water mixtures (with n = 1, 2 or 7) were determined at atmospheric pressure as a function of temperature. The excess, apparent, and partial molar volumes were deduced from experimental data, as well as fragilities, m*, and excess Gibbs free energies of activation of viscous flow exhibited predominantly negative deviation from ideality, with a minimum at approximately xw ~0.8 for all three systems, indicating strong hydrogen-bonding interactions. All three binary systems were found to be fragile, with [P4 4 4 4][C7COO] showing the smallest deviations in fragility with the addition of water. Mixing enthalpy values of the systems were exothermic over the entire composition range, having the following trend: [P4 4 4 4][C2COO] > [P4 4 4 4][C7COO] > [P4 4 4 4][C1COO].