Measurements have been made of the decay constant of thermal neutrons in water poisoned with boron and with the non-1/v absorber cadmium. An experimental method has been used in which proper spatial integration of the neutron flux enables data that are representative of the infinite medium to be accumulated without waiting for the establishment of a fundamental mode distribution. The measurements yield effective absorption cross sections in good agreement with presently adopted values. The change in effective absorption cross section with concentration of the dissolved cadmium (dσeff)/ (dN) has been determined for an infinite medium at 20°C. Two- and three-parameter fits of the decay constant yield −(0.32 ± 0.09) · 10−17 b cm3 and −(0.47 ± 0.10) · 10−17 b cm3, respectively, for the coefficient (dσeff)/ (dN). Earlier published measurements have resulted in two to five times larger values, whereas a published calculated value for Nelkin's model is −0.33 · 10−17 b cm3.