It has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment such as the interstellar medium. In recent years it has become clear that not only ion/radical-molecule gas-phase reactions, but also solid state reactions on icy dust grains play an important role in the formation of new species. In order to investigate the underlying processes, laboratory based experiments are needed to simulate surface reactions induced by photon (UV) processing or particle (atom, cosmic ray, electron) bombardment of interstellar ice analogs. Here, the latest research performed on SURFace REaction SImulation DEvice (SURFRESIDE), one of the ultra-high vacuum setups in the Sackler Laboratory for Astrophysics in Leiden is reviewed. The focus is on hydrogenation, i.e., H-atom addition reactions in interstellar ice analogs for astronomically relevant temperatures. We discuss how molecules form when CO and O₂ containing ices are exposed to thermal hydrogen atoms under fully controlled experimental conditions. Surface formation schemes for interstellar relevant species, such as solid methanol, water, and carbon dioxide are investigated and chemical links between molecular species in space are discussed.