Accurate determination of acousto-elastic natural frequencies in centrifugal compressors is important in order to avoid resonance events that may lead to machine failure. Compressors operating with CO2 at high pressures, especially near its transition to supercritical state, deal with a wide variation in density and speed of sound. Natural frequency behavior under these conditions is studied here. A finite element method (FEM) based coupled acousto-elastic solver has been developed to study the modal coupling and interactions between the impeller and the side-cavity modes for an idealized compressor geometry. Pressure in the side-cavities is increased up to a very high value of 20 MPa and existence of fluid-A nd structure-dominant acousto-elastic modes is observed. The variation of the natural frequencies of these modes with pressure exhibits contrasting trends as CO2 transitions from gaseous to supercritical state. © 2017 by ASME.