Ab initio theory with split-valence plus polarization function and larger basis sets at the self-consistent field (SCF) and second order Moller-Plesset (UMP2) levels was used to predict the geometries and spectroscopic parameters for the ground and first excited electronic states of the HS2, FS2, and ClS2 free radicals. The ground-state species are predicted to be bent and may be described as having a sulfur-sulfur double bond, a sulfur-substituent single bond, and an unpaired electron which is delocalized principally over the sulfur centers. The first electronic transition involves an (n,pi*) electron promotion which leads to a longer S-S bond, smaller bond angle, and greater localization of the unrepaired electron on the terminal sulfur atom. The adiabatic transition energies are predicted to be at similar to 6700, 13700, and 10600 cm(-1) for HS2, FS2, and ClS2, respectively. The theoretical results are in good agreement with the rather limited amount of experimental data available. These ab initio predictions provide critical information to aid in future experimental studies of the matrix isolation or gas-phase spectra of the thiosulfeno radicals.