The effect of pressure on the segmental dynamics in two symmetric blends of PMPS and PS is studied for pressures Lip to 250 MPa. In these blends, there is interplay between spinodal decomposition and glass transition, resulting in the enrichment of the high T-g component by the more mobile component. The distinctly different pressure sensitivities of PS and PMPS are used as fingerprints of the phase state, allowing for identification of the origin of the two dynamic processes arising from the PMPS segmental dynamics in PMPS-rich and PS-rich domains. Model calculations using a lattice-based equation of state lead to prediction of the phase diagram, as well as the effect of pressure on the critical temperature for the same PS/PMPS blend. The weak pressure sensitivity of the critical temperature (dT(c)/dP). compared to the two segmental relaxations, suggests that a transition to a thermodynamically miscible but dynamically heterogeneous state takes place for pressures above 300 MPa.