The dynamics in poly(ethyl methacrylate) (PEMA) was studied as a function of ternperature (in the range from 133.15 to 453.15 K) pressure (0.1 to 300 MPa), and molecular weight (2.0 x 10(3) and 1.59 x 10(4) g/mol) for frequencies from 3 x 10(-3) to 10(6) Hz using dielectric spectroscopy (DS). In addition, rheological studies were made within the temperature range from 323.15 to 383.15 K. The studies reveal four dielectrically active processes-alpha,beta, alpha beta, and "slow". Oil lowering the temperature or increasing the applied pressure the alpha beta process is transformed into alpha and beta processes. The relaxation strength of the beta-process decreases markedly with increasing pressure (both above and below the glass temperature, T-g) and for T > T-g, is accompanied by a complementary increase in relaxation strength of the alpha-process. The origins of the four dielectrically active processes are discussed in terms of the (i) apparent activation volume and (ii) values of the ratio of activation energies at constant volume and pressure. The latter allowed discussion of the relative contributions of thermal energy and volume to each of the dynamic processes. As a part of the paper. we derive the canonical set of equations that describe the effects of the thermodynamic variables P, V, Ton the average relaxation times and these are employed to aid the interpretations of the origins of the individual relaxation processes.