Аннотация:This concept article reviews application-oriented possibilities in the mechanistic investigation of photochemical reactions by electrospray ionization mass spectrometry (Photo-Chem-ESI-MS). A brief review of essential techniques for coupling photochemical reactions with ESI-MS online monitoring is presented. Representative customized advanced tools for “light on/light off” mechanistic studies of photochemical reactions aimed at the detection of specific intermediates are discussed. The design of dedicated Photo-Chem-ESI-MS setups is the focus of modern research and will enable more profound insight into the field.