Medium carbon steel gears untreated as well as Sursulf treated were tested in a back-to-back gear test rig. The failure of the test gears was by pitting. The contact stress-pitting life curves have been established for both untreated and Sursulf treated conditions. Sursulf treated gears exhibited low wear compared to untreated. Wear particle analysis of lubricating oil was carried out to analyse the nature of failure. Cumulative wear particle concentration at pitting limit has been suggested as a basis for predicting the onset of the failure in the gears. © 1989.