Dynamic combinatorial libraries that equilibrate under thermodynamic control and can be trapped kinetically when desired are key to creating complex systems that can mimic dynamic biological systems, such as the biochemical system of life. A much-sought-after feature is the ability to turn off the dynamic exchange of the system, in order to investigate a transient state away from thermodynamic equilibrium, and then turn on the dynamic exchange again. We describe here the first use of thiosemicarbazone exchange to form dynamic combinatorial libraries. The libraries were found to require a nucleophilic catalyst, or equilibrator, in order to reach thermodynamic equilibrium. This equilibrator approach adds a supramolecular level of control over the dynamic system and allows the dynamic exchange to be turned off by addition of 18-crown-6, which binds the equilibrator in a nonnucleophilic complex. The dynamic exchange can be restarted by addition of potassium ions that competitively bind 18-crown-6, thus liberating the equilibrator. The highly complex thiosemicarbazone-based macrocyclic libraries contain both [2]catenanes and sequence isomers, which can be distinguished by HPLC-MS/MS.