A new species, selenium monoiodide (SeI), was investigated for the first time at a high level of theoretical approach, SA-CASSCF/MRCI. The overall picture of all doublet and quartet (Lambda + S) states correlating with the three lowest dissociation channels and the associated Omega states provide reliable results to help understand the lack of experimental data on its transitions and to plan the investigation and determination of spectroscopic parameters. Transition probabilities were computed for the transitions X-2 X-1, A(1), A(2) - X-1, and A(2) - X-2, originated from the spin-forbidden 1 (4)Sigma(-) - X (2)Pi system, and the corresponding radiative lifetimes evaluated. (C) 2017 Elsevier B.V. All rights reserved.