Protic ionic liquids (PILs) have attracted much attention as a designer material for manifold applications but most promising used as fuel cell electrolytes due to containing active proton in the cationic structure. However, ultimate product of the fuel cell reaction is water, thus, it is very interesting to investigate the effect of water on the properties of PILs for applying as fuel cell electrolytes. In this study, PILs were synthesized through neutralization of a tertiary amine, diethylmethylamine (dema) with sulphuric acid (H₂SO₄) and trifluoroacetic acid (CF₃COOH). Binary mixtures of these PILs and water were prepared with a wide range of composition. The physicochemical and electrochemical properties of these PILs and their binary mixtures, such as density, viscosity, conductivity, thermal properties, ATR-FTIR, NIR, theoretical calculation and cyclic voltammetry (CV), have been studied in detail. The excess molar volume and excess viscosities were derived from density and viscosity and fitted to Redlich-Kister polynomial equation to confirm the presence and nature of interactions between PILs and water binary system. Analysis of FTIR-ATR and NIR spectra for the binary mixtures of [dema]CF₃COO and water also indicated that there is a strong interaction between ionic species and water. From quantum chemical calculations using density functional theory (DFT), the electron density distribution around the ion pair of [dema]CF₃COO is made and confirmed that there is a strong interaction between cation and anion not only due to strong hydrogen bond but also coulombic interaction. Conductivity of pure [dema]HSO₄ was lower than [dema]CF₃COO, however, conductivity of [dema]HSO₄ and water binary mixtures was higher than the binary mixtures of [dema]CF₃COO at room temperature when mole fraction of water was 0.90. From Walden plot (log Λ vs logη^-1), it was revealed that [dema]HSO₄ and its binary mixtures resemble good PILs, on the other hand, [dema]CF₃COO and its binary mixtures are poor PILs, presumably due to high ΔpK_a of [dema]HSO₄. Finally, electrochemical properties of PILs and their binary mixtures have been analyzed by CV measurements. It is found that there is a strong effect of water on the OCP of PILs. However, these PILs and binary mixtures could be useful as fuel cell electrolytes.