Diffusion-weighted MRI methods often contain variability and bias in diffusion parameters that are measured between sites, scanners, and vendors. There is a clear need for a calibrated diffusion phantom to help identify and mitigate these differences. Here we utilize a 7T spectrometer to characterize MR parameters of T1, T2, and the water self-diffusion coefficient in aqueous Polyvinylpyrolidone (PVP) solutions and their dependence on temperature and PVP concentration, without imaging confounds. An empirical formula is presented for use wtih PVP aqueous solutions as a calibrated diffusion phantom. Data show that aqueous PVP solutions are well suited as a MR phantom material.