One of the holy grails in catalytic research is the determination of the structure of the active site. Information on the catalytically active site is notoriously difficult to obtain as it concerns a small minority of states in a sea of other silent states. This silent majority of states obscures the action that takes place on the active states. Because the core hole localizes the final state, X-ray absorption spectroscopy (XAS) is a powerful local probe of the electronic structure and XAS provides detailed information on catalysts under working conditions. A major limitation in the present experiments is the fact that the signal from the majority of non-active sites overwhelms the details from the active sites.In this proposal I will develop a new idea to solve this problem and allow x-ray spectroscopy to unveil the nature of active sites. The idea is based on a detailed knowledge of the resonant x-ray emission (RXES) process that allows the detection of RXES spectra that are specific for the active state only. Model calculations predict an enhancement of the active sites over the silent sites of approximately a factor 50, allowing the clear detection of active sites above 1 % presence.This method will be suitable to study active sites in heterogeneous catalysts based on transition metal ions. However, the approach is also suitable to study transition metal active sites in homogeneous catalysis and biocatalysis. In addition, the proposed research will have an impact on first principles x-ray spectroscopy calculations in theoretical chemistry and theoretical physics.