The scandium-sulfur system was investigated from the perspectives of crystal and electronic structure and thermodynamics in order to describe more exactly bonding in scandium monosulfide and other NaCl-type compounds;The transition temperature of an order-disorder transition observed in Sc(,0.81)S (R3m to Fm3m), which occurs as a result of low temperature vacancy ordering, was determined to be 700(DEGREES)C. In order to test the hypothesis that the driving force for vacancy ordering is electrostatic, a group of ordered sublattices on the NaCl-type lattice (Fm3m) was systematically generated and a Madelung energy and configurational entropy were calculated for each sublattice assuming the ions to be Sc('2.48+) and S('2-) (charge neutrality). Mean field and pair interaction approximations were used to model long-range and short-range orderings, respectively. The electrostatic model is found to fail in predicting the short-range and long-range orderings of the type observed;The high temperature vaporization of ScP was investigated by mass spectrometric and target collection Knudsen effusion techniques in the temperature range 1767-2209K. The composition ScP(,1.00) vaporizes congruently to the gaseous species Sc, P, and P(,2). A temperature independent third law enthalpy of atomization ((DELTA)H(,atom,298) = 252.2 (+OR-) 2.8 kcal mole('-1)) a value (TURN)12 kcal larger than that reported for ScS which, in a rigid band interpretation, suggests that the additional electon in ScS occupies a slightly antibonding orbital;Nonrelativistic, nonself-consistent LAPW band structure calculations are reported for ScS. Analysis of the -character of the wavefunctions and the directional nature of the charge density suggest that there is considerable Sc-S (sigma)-type bonding, a covalent contribution, within valence band states and primarily Sc-Sc and to a lesser degree Sc-S (pi)-bonding within conduction band states;XPS and UPS measurements are reported for Sc(,2)S(,3) and several compositions Sc(,1-x)S (0.0 (LESSTHEQ) x (LESSTHEQ) 0.2). The UPS spectrum obtained for Sc(,1.01)S and the theoretical density of states derived from the ScS band structure are found to be in general agreement;The Sc and S 2p binding energies (XPS) obtained for the defect scandium monosulfides are very close to those found in the pure elements, suggesting covalent bonding. The Sc 2p energy region is also observed to have an interesting satellite structure which is not explained by the usual models, but resembles the W 4f region in Na(,x)WO(,3);('1)DOE Report IS-T-896. This work was performed under Contract W-7405-eng-82 with the Department of Energy.