Physicochemical properties of aqueous solutions of potassium 4-fluorobenzoyl-trifluoroborate salt have been studied to know the effect of concentrations and time on its stabilities because these properties are essential to predict and control chemical reactions and, to develop new substances with innovative applications. Here, pH, refractive index, density, conductivity and molar refractivity have been evaluated during 92 days and, the Ka a and pKa a for all solutions have been calculated. Salt concentrations and time show effects on the densities, pH and conductivities of the solutions while the refractive indexes and molar refractivity remain practically constant during the time studied. The infrared, Raman and UV-vis spectra of acid in gas phase and aqueous solution have been predicted by using hybrid B3LYP/6-311++G** ++G ** calculations. Intra-molecular halogen O-H...F - H...F interactions predicted for the acid in both media probably explain the decreasing observed in the conductivity values. Full vibrational assignments of acid and its scaled force constants in both media are reported. The decreasing observed in the f(nu BF3) ( nu BF 3 ) force constants of the three species in both media suggest that the trifluoroborate group plays an important role in the properties of salt. The predicted UV spectrum of acid in aqueous solution compared with the experimental of salt suggest that the anion, acid and salt species are present in aqueous solution, as evidenced by the physicochemical properties. Hence, the presence of three species in solution justify the variations observed in the pH, density, conductivity, force constants and positions of IR and Raman bands related to OH and BF3 3 groups.