The anoxic dissolution of troilite (FeS) in acidic medium has been investigated at 50 degrees C using batch dissolution experiments. Two different progress variables were followed during solid dissolution, i.e., the amounts of dissolved iron (n(Fe)) and formed hydrogen sulfide (n(H2S)). The C experimental studies performed at hydrogen ion concentrations ([H+]) ranging from 0.04 to 0.2 mol L-1 showed that anoxic dissolution of troilite is dependent on [H+]. The cumulative release of both Fe and H2S could be described by a diffusion-like rate law, with rate constants for Fe (k(fe)(p)) always greater than for H2S (k(H2S)(p)) The surplus of dissolved iron over formed hydrogen sulfide was quantified by the n(Fe):n(H2S) ratio, and ranged from 1.21 to 1.46, higher than the specific n(Fe):n(H2S) ratio of troilite bulk, i.e., 1. Rate constants are linearly related to the pH with a slope of 0.66 +/-0.23 (n(Fe)) or 0.63 +/- 0.13 (n(H2S)). The obtained results suggest that troilite anoxic dissolution is a process controlled by the diffusion of the reaction products across an obstructive layer, sulfur-rich layer (SRL), having a thickness that increases during reaction progress. The accumulation of H2S between the surface and the SRL, eventually leads to the mechanical destruction of this outer layer, a process that results an increased flux of reaction products. (c) 2005 Elsevier Inc. All rights reserved.