[en] The successive phase transitions (PT) of CsSrCl₃ which belongs to the family of the cubic perovskites of the type ABX₃ are investigated by inelastic neutron scattering. At first the acoustic phonon dispersion is measured in the cubic phase near the first PT. Outgoing there from it is possible to determine the sequence of the phonon mode condensation by evaluating the temperature dependence of the quasi-elastic and Bragg scattering at the zone boundary points (M, R, and X). From the results of neutron scattering it is obvious that the PT into the tetragonal phase is caused by a condensation of a M₃-mode and that the characterization of the successive PT's, proposed originally by Alexandrov et al. is the correct one. (author)