Polydimethylsiloxane/polyisobutylene blends are investigated using ultrasonic techniques (megahertz range), in the complete concentration range (0%-100%). The velocities of propagation and attenuations of shear and longitudinal waves are determined experimentally together with simultaneous optical microscopy observations. From the knowledge of the acoustical and physical properties of the separate polymers, the acoustical longitudinal parameters of the blend can be predicted successfully up to 40% volume concentration of the dispersed phase, with a model developed by the authors and especially adapted to wave propagation in such viscoelastic emulsions. For shear waves, two emulsion models (Palierne and Lee-Park) are used for predicting the viscoelastic moduli associated with the acoustical data at ultrasonic frequencies, with a good agreement. The case of composite droplets (40%-60%) has required to consider Friedrich's extension of Palierne's model. The ultrasonic method appears to be a very interesting tool for predicting phase inversion for polymer blends, as well as the evolution of the microstructure of the blend. Finally, this work provides an extension of the validity of emulsion models in the high frequency range.