Fluid catalytic cracking (FCC) produces (the feedstock for) a major part of the world's fuels, as well as chemical building blocks for, for example, polymers, pharmaceuticals, and specialty materials. ZSM-5 is the active ingredient in propyleneselective FCC catalyst systems and is stabilized or activated with phosphorus compounds. Despite this process being one of the largest-scale industrially applied catalytic processes, there is still considerable debate on the mechanism of activation, as well as on the interaction between phosphate and zeolite aluminum species. In this work, we use synchrotron-based powder XRD, neutron diffraction, and subsequent pair distribution function analysis to unequivocally corroborate the activation mechanism of phosphorus-based promotion in FCC catalysis and localize the phosphate groups inside the pore system of P-activated ZSM-5. We find local disorder in the zeolite T-O coordination, which could not be observed with traditional XRD analyses. Furthermore, we support these experimental findings with full periodic quantum-mechanical modeling (QMM) of the highly relevant, but often overlooked, combination of dealumination by hydrolysis (steaming) and phosphatation of the zeolite framework. We thereby show that phosphate can react with partially dislodged aluminum species that remain stable and are still tethered to their original framework position. Finally, by assessing all available literature postulations by the same periodic QMM and comparing them energetically with our obtained results, we can conclude that by accounting for the highly relevant inclusion of steaming prior to phosphatation, the two models resulting from this work rank among the three most relevant remaining models. This combined experimental and theoretical work fundamentally explains the activation and promotion mechanism of one of the world's most applied chemical processes?propylene-selective FCC.