99Tc and 129I are important contributors to risk assessment due to their long half-lives and high mobility as aqueous anionic species. We analyzed 99Tc and 129I in groundwater samples in and near 11 underground nuclear tests and in melt glass and rock samples retrieved from the Chancellor test cavity, Nevada Test Site. The 129I/127I ratio ranges from 10−3 to 10−6 in cavity water and 10−4 to 10−9 in satellite wells. The 99Tc concentration ranges from 3 to 10−4 Bq/l in cavity waters and from 0.3 to 10−4 Bq/l in satellite wells. Downstream migration is apparent for both radionuclides. However, it is affected by both retardation and initial distribution. In-situ 99Tc and 129I Kds calculated using rubble and water concentrations are 3 to 22 ml/g and 0 to 0.12 ml/g, respectively, and are suggestive of mildly reducing conditions. 129I distribution in the melt glass, rubble and groundwater of the Chancellor test cavity is 28%, 24% and 48%, respectively, for 99Tc, it is 65%, 35% and 0.3%, respectively. Our partitioning estimates differ from those of underground tests in French Polynesia, implying that fission product distribution may vary from test to test. Factors that may influence this distribution include geologic conditions (e.g., lithology, water and CO2 content) and the cooling history of the test cavity.