The age of U235 fission neutrons to indium resonance energy has been measured in a number of lattices of Th-H2O and Th-Al-H2O and compared with theoretical calculations. As a check on the procedure similar measurements were made in pure water yielding a result of 27.0 ± 0.9 cm2. In addition, the ratio of the age of U233 fission neutrons to the age of U235 fission neutrons in water was found to be 0.948 ± 0.033.