Results of the amplitude-dependent dynamical behavior of commercial poly(vinyl chloride) are presented. Both the loss factor (internal friction) and the storage Young’s modulus are strain amplitude dependent when measured between room temperature and approximately 400 K at a frequency of 50 kHz. The data are interpreted in terms of a hyperbolic sine stress dependence of the strain rate, including an effective stress. The activation volume was found to depend on temperature, with values much larger than those given by the usual mechanical tests (tensile, creep, or stress relaxation). It is suggested that these nonlinear effects are related to the cu relaxation.