Molecular simulations of the siting locations and energetics of Mo(CO)(6) and C6H6 cosorbed in faujasitic zeolites Na(n)FAU (n = 0-96, Si/Al = 100-1) have been presented in combination with Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The in situ DRIFTS technique was found to be an efficient tool to monitor the cosorption at low coverage as well as the reaction of Mo(CO)(6) and C6H6 under thermal activation within the void space of the Na(n)FAU zeolites. The molecular simulations are based on Monte Carlo calculations using the grand Canonical ensemble and are derived from a suitable zeolite-metal carbonyl-hydrocarbon potential set. From the present experimental and theoretical results as well as earlier experiments related to the reagents sorbed alone, a coherent picture of the cosorption and chemical behavior of Mo(CO)(6) and C6H6 within the void space of the faujasitic zeolites has been drawn as a function of the aluminum content. In siliceous faujasite (Si/Al = 100) the Mo(CO)(6) and C6H6 molecules are randomly distributed within the void space and the molecular motions approach the rapid isotropic limits of liquids. The chemical behavior upon thermal activation is found to be analogous to that observed in solution. In Na(56)FAU (Si/Al = 2.5) the reagents are trapped in well-defined sorption sites in close proximity. Upon gentle thermal activation a fast reaction occurs to form Mo(CO)(3)(eta(6)-C6H6) inside the supercage through a concerted mechanism including the electrostatic field and the basicity of the framework oxygens. In Na-(85-96)FAU (Si/Al = 1.25, 1) the Mo(CO)(6) and C6H6 molecules are not encapsulated in close proximity. Mo(CO)(6) reacts thermally in the void space like in the absence of added C6H6 to lose sequentialy three CO ligands and form predominently a Mo(CO)(3)(O-z)(3) species in which the three vacant coordination sites are occupied by three O-z framework oxygens.