AbstractGamma rays from the fission product nuclides140Ba and140La are frequently used for monitoring fission rates in neutron activation detectors consisting of fissionable materials. Systematic errors that can occur when the 1596-keV line of140La is taken as the monitor line can be avoided by using the 537-keV line of140Ba. The gamma-ray emission probabilities per decay of140Ba and140La were redetermined for the more abundant lines with a relative uncertainty of about±1% at the 68% confidence level.