The study of the equation of state of caesium bromide is based on the method adopted by Verma and Dayal for sodium chloride. It was observed that, corresponding to the volume ratio v_T/v₀ >or= 1.18 for some wave vectors, some of the frequencies obtained by solving the secular determinant were imaginary. As this ratio was smaller than the maximum volume ratio 1.27 for caesium bromide at the melting point, it implied that the usual form of the potential energy function leads to the instability of the lattice before it melts. All other available forms of the potential energy functions were also tried but did not improve the results. It was found that real frequencies could be obtained by augmenting Born and Mayer's potential by a term which is three times larger than the normal van der Waals and dipole-quadrupole terms.The new potential energy function has been used further for evaluating the following four quantities. cohesive energy, melting point and the temperature variations of thermal expansion and compressibility. The agreement with experiment is good for the first and reasonable for the other quantities.