Relativistic coupled cluster theory is used to determine accurate electric field gradients in order to provide a theoretical value for the nuclear quadrupole moment of La139. Here we used the diatomic lanthanum monohalides LaF, LaCl, LaBr, and LaI as accurate nuclear quadrupole coupling constants are available from rotational spectroscopy by Rubinoff [J. Mol. Spectrosc. 218, 169 (2003)]. The resulting nuclear quadrupole moment for La139 (0.200±0.006 barn) is in excellent agreement with earlier work using atomic hyperfine spectroscopy [0.20 (1) barn]. © 2007 American Institute of Physics.