This review covers the complex lower temperature chemistry of iron sulfides. Fe and S chemistry is examined in the context of iron sulfide formation processes and their kinetics with particular reference to marine systems. Three themes are followed. First, the equilibrium dynamics of the aqueous iron, sulfide and iron sulfide systems are re-evaluated. Second, the importance of carefully distinguishing between kinetics and equilibrium thermodynamics in understanding iron sulfide chemistry is outlined. Finally, the consistency of apparently conflicting reports on iron sulfide chemistry in terms of the equilibrium and kinetics approaches is discussed.