A new experimental setup for in situ/operando investigations of redox reactions is introduced. This setup, in combination with ultra-high vacuum (UHV) methods from the field of surface science, provides completely new possibilities to investigate electrochemical redox reactions. Two types of cells are distinguished conceptionally: in the permeation configuration, the working electrode is electrochemically polarised on one side of a membrane (entry side), leading to atomic hydrogen uptake, and allowing proton and electron exchange between the entry and the other side (exit side) of the membrane. Here it is found that the applied potential on the entry side shows a 1:1 correlation with the measured potential on the exit side. The concept of the "window" cell requires ultra-thin, electron transparent "windows," such as single layer graphene, for X-ray photoelectron spectroscopy or X-ray transparent silicon nitride "windows" for X-ray absorption spectroscopy. In this case, the solid/liquid interface can be directly probed under applied potentials. In both configurations, the applied potential is measured with a palladium hydride reference electrode, with so far unseen precision and long-term stability. The cell design is constructed with regard to transferability within a UHV system, allowing sample preparation, and a modular construction, allowing a straightforward changeover between these two configurations. As a first application, an approach based on atomic hydrogen is presented. Further application concepts are discussed. The setup functionality is demonstrated by the example of in situ/operando investigation of the palladium oxide reduction.