MOFs have been used as supports for various Pd species, most often NPs, and used as very efficient catalysts in different cross-coupling reactions. In some instances, spectacular reactivity under very mild conditions has been achieved. This is the case for the Suzuki-Miyaura cross-coupling reaction, where Pd@MOFs such as MOF-253, MOF-5, 51 IRMOF-3, 52 MIL-53-NH2, and MIL-101 have been extensively investigated as catalysts. In particular, we reported the use of Pd@MIL-101-NH2(Cr) as a catalyst for the coupling of highly functionalized arylboronic acids and aryl halides. The catalyst consisted of evently distributed Pd NPs, with an average size of 2.6 nm. Loadings of palladium as low as 0.01 mol% could be used, under very mild conditions (20 °C) with H2O/EtOH mixtures as the solvent, resulting in TONs of up to 10000. In contrast, similar palladium nanoparticles immobilized on other supports require very harsh reaction conditions. With the aim of understanding the distinctive reactivity in all these system, we have now investigated the structure of the Pd species formed in the Suzuki-Miyaura cross-coupling reactions mediated by Pd@MIL-101-NH2(Cr) by means of electrospray ionization mass spectrometry, ESI(-)-MS, density functional theory (DFT) calculations, and 19 F NMR spectroscopy. In this report, we have identified trimeric and tetrameric palladium oxidative addition species as key intermediates in Suzuki-Miyaura cross-couplings catalyzed by Pd@MOF. Importantly, it was found that these intermediates undergo a very facile transmetallation. These species, which have never been proposed before, have also been identified when using other ligandless systems, such as Pd(OAc)2 in absence of strong donor ligands.