The thermal value of eta for U233 and U235 has been determined in a series of experiments on unreflected homogeneous aqueous solutions of the two isotopes. These experiments also yield a value for the neutron age and the limiting concentrations of the fissile isotope in the aqueous solutions for infinite volumes. Auxiliary experiments, establishing limits of error, testing certain aspects of the theoretical model employed, and experimentally determining the parameters in the critical equation, have been performed. Experiments performed with 27-in.- and 48-in.-diam spheres, and 5-ft- and 9-ft-diam cylinders have yielded consistent values of eta. Measurements of the nonleakage probability in cylindrical geometry have given values consistent with those predicted by a two-group model in which the theoretical value of the age was used. Within the experimental error no differences were found in the ages of fission neutrons for U233 and U235. The average thermal values of eta determined are: for U233, 2.292 ± 0.015 and for U235, 2.076 ± 0.015. The 2200 meters/sec values are the same since the g-factors for eta are unity. The value of the neutron age to the indium resonance energy for U235 fission neutrons in water was found to be 25.6 ± 1.3 cm2. The minimum U233 and U235 critical densities for these nitrate solutions were found to be 11.25 ± 0.10 gm/liter and 12.30 ± 0.10 gm/liter for U233 and U235, respectively.