This work investigates the stability and reproducibility of brain-mimicking microfiber phantoms. These microfibers were produced by co-electrospinning (co-ES) and characterized by scanning electron microscopy (SEM). Grey matter (GM) and white matter (WM) phantoms were constructed from random and aligned microfibers, respectively. MR data were acquired from these phantoms over a period of 17 months. SEM images reveal that there were some changes in the pore size and porosity of co-ES fibers over a period of 30 months. MR measurements showed variations within the limits expected for intra-scanner variability, thereby confirming the phantom stability over 17 months.