AbstractA fast coincidence technique was employed to measure the relative prompt-neutron yields per fission of U233, U235, Pu239, and Pu241induced by thermal neutrons. The values obtained wereν23/ν25= 1.02 + 0.01,ν49/ν25= 1.23±0.01, andν41/ν25= 1.295±0.02, whereν23,ν25,ν49, andν41designate the prompt-neutron yields of U233, U235, Pu239, and Pu241, respectively.