Abstract Aggravated corrosion conditions caused by FeS sol ids and by mixed air plus H2S interactions are responsible for some of the highest equipment failure rates in the petroleum industry. laboratory studies have shown the existence of a transient, ionic, sulfur species which is formed when air and H2S react in an aqueous environment. Iron sulfide catalyzes the reaction to form the species and perhaps also its decay. In addition, the dispersed FeS particles cause an aggravation of the corrosion independent of its catalytic effect on the air/H2S reaction. Furthermore, deposits of FeS cause under-deposit corrosion when air is mixed with sour fluids. A new organic sulfo-phosphate composition has been found which effectively inhibits the most severe combinations of these conditions. It also inhibits cracking in H2S environments. INTRODUCTION Even after a considerable number of years of experience in this laboratory working on inhibition of H2S-caused corrosion in petroleum equipment, there remain a significant number of puzzling questions about the corrosion effects observed in the field. The work reported here covers experiments designed to try to elucidate the features of some of these puzzling occurrences and identify at least some of the causes. In addition, we have developed a corrosion inhibitor which is quite different from the conventional kind and is much more effective under the conditions described. Specifically, we have found that oxygen and iron sulfide solids are significant contributors to the corrosion process such that rates as high as several hundred MPY (mils penetration per year) have been observed both in the field and in the laboratory.