A phantom for use with magnetic resonance imaging (“MRI”) and, in particular, for calibrating quantitative diffusion MRI is provided. In general, the phantom includes a solution composed of a solvent that has diffusivity value higher than that of water, and a solute that when added to the solvent reduces the diffusivity of the solution. By varying the combined concentration of the solvent and solute, the diffusivity of the solution can be controlled to fall within a range of diffusivity values found in biological tissues in a variety of different physiological conditions or tissue environments.