On July 16, 1945, the Trinity nuclear test exploded in the desert near Alamogordo, New Mexico. A variety of new diagnostic experiments were fielded in an effort to understand the detailed performance of the nuclear device. This paper describes a series of radiochemical experiments that were designed to measure the efficiency and neutron fluence of the test. These experiments, and the scientists who led them, laid the foundation of weapons radiochemistry for decades to come.