Acquiring a full understanding of what exactly happens when two molecules interact and react with each other is of fundamental interest to chemistry. This proposal touches upon a quest that has been one of the holy grails in the field of molecular reaction dynamics since it was established in the 1960’s: to study a chemical reaction under completely controlled conditions. We propose to study benchmark reactions such as OH + O → O2 + H with complete quantum state and kinetic energy control before the reaction, and complete information on product state and recoil energy after the reaction. This is accomplished using the combination of Stark and Zeeman deceleration technology to prepare the reagents, and velocity map imaging to probe the reaction products. This combination of techniques has recently revolutionized inelastic scattering experiments, as demonstrated by several papers of our group in Science and Nature. Here, we propose to apply this methodology for the very first time to reactive scattering. If successful, this will open up an entirely new research avenue, with implications that reach out much beyond the OH + O reaction alone. It will provide new means to give unprecedented detailed views on “the chemical act” with extreme resolutions and in unexplored energy regimes approaching zero Kelvin. It will give theoreticians the input that is necessary to describe these fundamental mechanisms that are at the basis of all chemistry.