Fractional electron capture probability ratios of 125 I were measured using metallic magnetic calorimeters (MMCs). Due to the 4π geometry of the source embedded in the absorber, detection efficiency of nearly 99% was observed for K, L, M, and N capture events. The fractional capture probabilities were calculated from peak area ratios. Keywords Electron capture probabilities • Cryogenic detectors • Decay data• Decay energy spectroscopyDetermination of fractional electron capture probabilities of 125 I using Metallic Magnetic Calorimeters.