Twenty two lowest-lying electronic states of the species SiB are described theoretically using the internally contracted multireference configuration interaction approach and natural orbitals generated from a state-averaged density matrix. Correlated consistent valence quadruple-zeta (cc-pVQZ) atomic functions are used in the expansion of the one-electron basis. Potential energy curves are presented for all the states as well as a description of the electronic structure characterizing the most relevant ones. Dissociation and excitation energies and dipole moment functions complete the electronic structure description. The solution of the radial nuclear equation allowed the determination of vibrational energies and vibrational and rotational constants. For the ground state (X (4) Sigma(-)), R-e = 1.918 Angstrom and D-e = 3.27 eV. The first excited quartet (A (4) Pi) is located 2.60 eV (T-e) higher in energy and has a slightly shorter equilibrium distance (1.811 Angstrom) and a smaller D-e, 1.00 eV; the transitions X (4) Sigma(-) - A (4) Pi are expected to fall in the visible (green-yellow) region. Higher-lying (4) Pi states show very noticeable changes in the potential function due to avoided crossings. Within about 1.5 eV from the ground state there are located five doublet states; the lowest one a 2 Pi with T-e = 0.73 eV, R-e = 1.845 Angstrom, and D-e = 2.54 eV. For selected states, transition dipole moments, transition probabilities, and radiative lifetimes are also presented. Crossings of various states and the energetic closeness of others are expected to play non-negligible perturbative effects in the spectra. The global picture of the electronic states presented will certainly be an important aid to experimentalists in the spectroscopic investigation of this species. (C) 1997 American Institute of Physics. [S0021-9606(97)03141-3].