MetroMMC is a European metrology research project that aims to determine electron capture (EC) decay data on pure EC nuclides as well as EC nuclides with additional decay branches, both experimentally and theoretically. Metallic magnetic calorimeter (MMC) detectors have the potential to measure electron capture decay data in low to medium-energy transitions with unprecedented precision. MMCs offer excellent energy resolution, enable low energy thresholds as well as near 100% detection efficiencies for electrons and photons in the decay energy spectroscopy (DES) measurement mode. Nickel-59, one of the radionuclides chosen as a part of the MetroMMC project, decays purely byEC. It is used for dating of the solar system, meteorites and in geology. In this poster, the measured fractional electron capture probabilities of 59Ni embedded in the metallic absorber coupled to the MMC sensor will be presented. Two different source preparation techniques adapted for the measurement will also be discussed: one prepared by electroplating and the other one by micro drop dispensing on gold nanofoam. The total energy spectra obtained from both sources are compared with each other, and the fractional electron capture probabilities are compared with those available in the literature and from the BetaShape program. The measurement for another EC radionuclide, 51Cr, is also underway and can be discussed.