A symmetric critical isotopic mixture of protonated and deuterated poly(ethylene-propylene) (PEP) has been quenched from the homogeneous state (Tc = 93 °C) to temperatures ranging from 45 to 89 °C. As reported for a previous isotopic polybutadiene mixture, the classical light scattering results from this sample fail to reduce to a universal curve by the scaling of time and length based on scaling parameters extracted from linear analysis. The light scattering results confirm the mean-field nature of the thermodynamic aspect of the Cahn theory, producing initial temperature-dependent heterogeneity lengths that are quantitatively anticipated by the de Gennes-Pincus-Binder modifications. Calculation of the effective diffusion coefficient based on independent measurements of the cooperative diffusion coefficient leads to an order of magnitude greater value than is obtained from the linear analysis of the time-dependent light scattering data. This disparity calls into question the conventional assumption of mean-field van Hove dynamics, which ignores hydrodynamic interactions, in the treatment of spinodal decomposition in polymer mixtures. Rheological measurements also demonstrate that, within the limits of our equipment, spinodal decomposition is not discernible through dynamic-mechanic experimentation.