Internally and externally plasticized poly(vinyl chloride) (PVC) samples have been investigated by means of different techniques concerning volumetric, elastic and dynamic properties and the results have been compared to those obtained from pure PVC. In all the cases, the glass transition temperature of PVC is markedly lowered. However, this decrease is more important for the external plasticization. This is confirmed by the position of the dielectric loss peaks of the alpha relaxation. Independently from the changes in temperature, the shape of the relaxation function is modified only in the case of internally plasticized PVC. We suggest that this change in the relaxation function could be related to the small variations produced in the wide angle diffraction patterns of the samples. We can observe that the plasticization effect is linked to a decreasing of the intensity of the dielectric beta relaxation process but no important changes are produced in the activation energy of this process.