The absolute thermal value of η for U233 and U235 was measured directly by a method of total absorption which involves relative counting of manganese bath activations and some minor corrections. A thermal neutron beam (defined by cadmium difference) is introduced in the center of a one-meter-diameter sphere filled with a dilute solution of manganous sulphate in water. The beam is first made to activate the bath directly, then it is totally absorbed in the fissionable sample whose fission neutrons then activate the bath. The ratio of the two activities is equal to η except for small corrections. The results obtained for η corrected to 2200 m/sec were, for U233, 2.296 ± 0.010; and for U235, 2.077 ± 0.010.