The influence of rapid thermal annealing (RTA) and sulfur passivation of InSb on the electrical characteristics of InSb metal-oxide semiconductor (MOS) structures are studied. The parameters of interest are the current conduction mechanism, electric breakdown voltage and oxide fixed charges. This study shows that RTA for temperature up to 350 degrees C improves the electrical properties, For temperatures higher than 350 degrees C, electrical properties degrade. Passivation of InSb improves the thermal stability of MOS structures. The introduction of Sb atoms (at lower annealing temperature) and In atoms (at higher annealing temperature) in unpassivated structures and the formation of stronger S-In bonds in the passivated structures were used to explain the observations.