The ionic association constant (K) of N-methyl pyridinium iodide (NMPI) ion in several ethanol-water mixtures were determined by the combination of UV spectroscopy and conductance measurements using the Shedlovsky function as a correction factor. The measurement of electrical conductance and UV absorption were performed in 95, 90, 80 and 60 volume percentages of ethanol in the solvent mixture at 15, 25, 35 and 45 $({pm}0.1)^{circ}C$. The ion size parameter $(r_A+_D-)$ and the dipole moment $({mu}_A+_D-)$ of NMPI ion were obtained from he linear plots of ln K vs. (1/D) and (D-1)/(2D+1), respectively. These ${mu}_A+_D-$ values were in good agreement with the values of transition moment calculated from the equation, ${mu}_{nm}=6.5168{ imes}10^{-2}{ imes}({varepsilon}_{max}{frac{ar{ u}_{frac{1}{2}}}{ar{ u}_{max}})^{frac{1}{2}}$ (Debye) which we have derived. The thermodynamic parameters indicate (1) that the water dipoles have an ordered rearrangement around the dipolar NMPI ions rather than the configuration existing in bulk free waters; and (2) that the equilibrium state between NMPI ion and its component ions are controlled by entropy.