A series of spin-orbit configuration interaction calculations has been carried out for the BiSe and BiTe molecules and analyzed in comparison with data obtained earlier for the isovalent BiO and BiS systems. An avoided crossing caused by the spin-orbit interaction between the X (2)Pi and A (4)Pi electronic states is shown to have a decisive effect on the lower-energy spectrum in each case. Irregularities in the X-2 3/2 state vibrational manifold occur as a consequence of this nonadiabatic interaction, and the v vibrational number for the onset of these perturbations is found to gradually decrease in going from BiO to BiSe, in agreement with experiment. In BiTe the shape of the X-2 potential curve is so altered by the avoided crossing that its minimum becomes shifted to a significantly larger distance than for the X-1 state, unlike the case for BiSe or the lighter Bi chalcogenides. This characteristic appears to be the root cause for the fact that the X-2 state has not yet been found experimentally in the BiTe spectrum, despite careful searches in the expected energy range. Radiative lifetimes have also been calculated for the low-lying states of both the BiSe and BiTe molecules, and these results are found to be consistent with experimental observations. (C) 2004 American Institute of Physics.