The segmental relaxation process in atactic and isotactic polypropylene (PP) is studied over the temperature (258-323 K) and pressure (0-300 MPa) ranges by means of dielectric spectroscopy. The combined dielectric and equation of state data provide a means of disentangling the relative influence of thermal energy and density on the glass "transition". Thermal energy is the controlling parameter of the segmental dynamics in both atactic and isotactic PP as inferred by the values of the ratio of the apparent activation energies at constant pressure and volume (0.72 and 0.63, respectively). Despite the common origin of the segmental dynamics, the relaxation times of the isotactic polymer exhibit higher pressure sensitivity due to the increasing crystallinity on pressurization.