The ultrasonic velocity and their related thermoacoustical parameters were successfully employed to understand the miscibility and molecular interactions in polymer blend. Ultrasonic velocity, density and viscosity in polyvinyl chloride with polyvinyl acetate in tetrahydrofuran and the thermoacoustic parameters viz., adiabatic compressibility, molar sound velocity, molar compressibility, expansion coefficient, acoustic impedance, van der Waal’s constant and internal pressure have been computed from the experimental data. The measurements have been carried out by using pulse echo overlap technique (PEO) at frequency of 4MHz at temperature 313K. The variation of u.s. velocity and other thermo-acoustical parameters shows nonlinear variation with molar concentration which suggest immiscibility or semi compatibility among the component polymers. Two immiscible polymers are need to be compatibilized in order to be used in commercial applications. The nature of solvent/polymer/polymer interaction and the effect of concentration on the molecular interaction of polyvinyl chloride and polyvinyl acetate in tetrahydrofuran have been studied.