Environmental transmission electron microscopy is a proven characterization technique for materials science. Exposing samples to their operating environment can provide new insight into their working state. In the case of photocatalysis, the operating conditions include exposure to photons as well as a gaseous environment. By adding optical coupling to the transmission electron microscope (TEM) it is possible to gain insight in the fundamentals of their reaction mechanisms, chemical behavior, structure and morphology before, during and after reaction using in situ investigations. Typically, photocatalysts work in gaseous or liquid atmosphere upon light illumination. Here, the aim is to reproduce their working conditions in situ. The Environmental TEM (1) allows exposing specimens to a controlled gas atmosphere, thus implementation of in situ sample illumination is needed. For this purpose, two novel specimen holders capable of shining light onto samples inside the TEM and to probe the sample using visible light spectroscopy techniques were designed and constructed (2). The holders were used to characterize photoactive materials in a simulated working environment and employed in the analysis of various photoreactive materials and structures. Novel information on the behavior of such materials during reaction was acquired in a reproducible fashion. In a wider perspective, the aim is to build a versatile experimental platform inside the microscope that allows electron microscopy under nonconventional TEM conditions and new kinds of in situ spectroscopy. As an example of the a photoinduced reaction, the images below show metal impregnated GaN:ZnO before and after visible light illumination for 5 hours at λ = 405 nm in the presence of H2O but in absence of electron beam. After illumination, Pt particles are clearly visible on the substrate surface.