~Received 28 March 2002; published 16 September 2002! The a relaxation process of the fragile glass former salol is investigated in the T-P domain by means of photon correlation spectroscopy. We find that a time-pressure superposition principle is obeyed for the relaxation function in addition to the time-temperature superposition. The behavior of the relaxation time is studied by using an extended version of the Adam-Gibbs model including the pressure dependence. The excellent conformity of the equation describing the bidimensional surface t(T, P) to the experimental data provides a positive check for this model, here verified on photon correlation measurements. The same model gives a rationale of the phenomenological expressions recently introduced to describe the changes in the slow dynamics induced by varying both temperature and pressure. These findings suggest that the reduction of configurational entropy actually guides the liquid toward the glass transition. The study of the dynamics of glass forming systems is of great importance for both basic and technological reasons. Indeed, the glass formation is crucial in the processing of food, in the technology of many daily used materials, from window glasses to polymers and composite materials, for the stabilization of biochemicals, for understanding structure and dynamics of proteins, etc. @1#. Although the molecular processes underlying the glass formation are not fully understood, the supercooling of liquids and the glass formation is known since the ancient age, showing two most significant common features: ~i! the stretched-exponential shape of the a relaxation function and ~ii! the steep increase of the a relaxation time t approaching the glass transition. The glass transition can be induced both by decreasing temperature and by increasing pressure, the first path being usually preferred due to the high pressure ~of the order of MPa! required for inducing dynamical changes similar to those obtained within few tens of degrees changes of temperature. For this reason, the glass formation is widely studied by measuring the temperature behavior of the a relaxation function of supercooled liquids. On the other hand, relatively few studies of pressure dependence of the a relaxation have shown to give a deeper insight into the nature of the transition. The high-frequency dynamics under compression has been investigated by means of both light scattering @2# and incoherent neutron scattering @3# in order to test the mode coupling theory @4# and study the equation of state of liquids @5#. The low-frequency dynamics under pressure has been investigated by dielectric spectroscopy to study the time-pressure scaling law @ 6‐8 #, and the fragility of different glass formers @9,10#. In the same low-frequency regime, photon correlation technique has been also used to test both the scaling of the a relaxation and the pressure dependence of fragility @11#, showing the traces of some universal behavior. This phenomenological picture is still waiting for a compre