In combining spin-and symmetry-resolved photoemission, magnetotransport measurements and ab initio calculations we detangled the electronic states involved in the electronic transport in Fe 1Àx Co x ð001Þ=MgO=Fe 1Àx Co x ð001Þ magnetic tunnel junctions. Contrary to previous theoretical predictions , we observe a large reduction in TMR (from 530 to 200% at 20 K) for Co content above 25 atomic % as well as anomalies in the conductance curves. We demonstrate that these unexpected behaviors originate from a minority spin state with Á 1 symmetry that exists below the Fermi level for high Co concentration. Using angle-resolved photoemission, this state is shown to be a two-dimensional state that occurs at both Fe 1Àx Co x ð001Þ free surface, and more importantly at the interface with MgO. The combination of this interface state with the peculiar density of empty states due to chemical disorder allows us to describe in details the complex conduction behavior in this system. Since the discovery of giant magnetoresistance (GMR) in spin valves in 1988 [1], a new branch of physics referred to as spintronics has considerably developed. The discovery of the large tunnel magnetoresistance (TMR) in 1995 [2], the prediction of the spin-transfer mechanism in 1996 [3,4], and the demonstration of spin-dependent coherent tunnel-ing in MgO-based epitaxial MTJs in 2001-2004 [5–10], have largely contributed to developments in this field. Currently, a number of new areas are being explored, such as rf oscillators, devices and memories based on the spin-transfer-torque effect, electric field assisted switching , magnonics, or spincaloritronics [11]. In addition, industrial-scale devices such as magnetic recording heads already use the exceptional electrical properties of GMR and TMR. The technology transfer from research to industry continues today, with MRAM demonstrators based on MgO-based MTJs [12] and rf oscillators using spintronics devices. While commercialization as well as broad utilization into various areas of research has been rapid for spin-tronic devices, in many cases a full understanding of the underlying physics is lacking. MgO-based MTJs with FeCo or FeCoB electrodes are a striking example of this situation. FeCoðBÞ=MgO=FeCoðBÞð001Þ multilayers, fabricated by molecular beam epitaxy (MBE) or sputtering deposition are widely utilized for their high spin current injection efficiency and exceptional electrical sensitivity to any change in the magnetic configuration of the electrodes. Because of the huge TMR predicted by ab initio calculation for the equimolar and B2 ordered Fe 0:5 Co 0:5 alloy and for pure bcc Co (1000%–6000% at 0 K [13]), bcc FeCo(001) electrodes are now extensively used in MTJ fabrication. However, the situation is not so clear regarding the reported results. First, large TMR were actually obtained on MBE grown Fe=bcc Co=MgO=Co=Feð001Þ [14]. However, a heating of the whole stacking up to 250 C during 30 minutes suggest a possible alloying between Fe and Co. On the other hand, contrary to expectations, epitaxial Fe 0:5 Co 0:5 =MgO=Feð001Þ and Fe=MgO=Feð001Þ MTJs exhibit the same TMR [15]. It should be noted that the B2 order assumed in Ref. [13] is not observed. Finally, reported TMR of sputtered FeCo=MgO=FeCoð001Þ MTJs present a nonmonotonic dependence as a function of the Co concentration with a maximum around 25% of Co [16]. The detailed effect of Co alloying into Fe on the spin-dependent tunneling remains therefore obscure. In this Letter, we explain quantitatively the unexpected transport properties observed in FeCo=MgO=FeCoð001Þ MTJs. We demonstrate that transport measurements alone are not sufficient to complete the current understanding of this system, and that spin-, symmetry-, and angle-resolved photoemission, together with DFT calculations taking into account the chemical disorder, offer a unique path to probing directly the tunneling electrons. We use a specific photoemission experiment to untangle the different Bloch waves responsible for the conduction along (001) as a function of their symmetry (Á 1 or Á 5) and spin state (majority " or minority #), in contrast to standard transport measurements where all these contributions are mixed. bcc MgO-based MTJs with Fe 1Àx Co x ð001Þ electrodes were grown by coevaporation using MBE. The epitaxial relationship, growth mode, and surface flatness were controlled using reflection high energy electron diffraction (RHEED). In addition, the evaporation rates of the Co and Fe sources, and consequently the alloys stoichiometry, were accurately controlled by recording the intensity