Small-angle neutron scattering measurements are presented as a function of temperature and composition for homogeneous mixtures of poly(methyl vinyl ether) and D2O. The experimental data are analyzed to give values of the second-order compositional derivative of the Gibbs energy and the Ornstein-Zernike correlation length. From the experimental data the LCST spinodal temperatures are estimated and values for the parameters in a temperature- and composition-dependent extended Flory-Huggins (F-H) interaction function are determined. Using the extended interaction function the predicted miscibility behavior is in qualitative agreement with the experimental data, and importantly, the remarkable bimodal phase behavior is predicted. In the composition interval 0.75 = wPVME = 0.85 the Ornstein-Zernike correlation lengths follow the mean field sum rule, i.e., ¿2 ˜ [(¿2¿G/(NkT))/¿22]-1. However, in the composition range 0.1 = wPVME = 0.7 the correlation lengths are still proportional to the second-order compositional derivative of the Gibbs energy but the data cluster on separate power laws with distinct exponents. Finally, the experimental data do not support the existence of a stable molecular complex at the investigated temperatures and compositions. Even at the lowest investigated temperature the energy required to induce typical Ornstein-Zernike-like concentration fluctuations is smaller than the thermal energy. Therefore, in the investigated temperature interval it must be concluded that the strength of the specific interactions between D2O and PVME is too weak to speak about complex formation.