Herein, we study the dielectric properties of various mixtures susceptible to be used in manufacturing of reference inhomogeneous breast phantoms dedicated to the experimental validation of microwave breast imaging systems in the 0.5-6-GHz frequency range. Particularly, we investigate the stability over time and temperature of these properties and their reproducibility for a given mixture, as well as the ability of some mixtures to mimic the various breast tissues, i.e., to show dielectric properties close to that given by one-pole Debye models that describe the mean relative dielectric permittivity of various tissue types.