The rates of reactive absorption of H2S into aqueous solutions of ferric nitrilotriacetic acid, at T = 303 K and pH = 4.5, studied in a flat-interface stirred cell, appear to be auto-catalyzed by freshly precipitated sulfur particles. These auto-catalytic effects, which are more prominent at higher ferric chelate concentration, seem to involve particle-to-interface adhesion phenomena. A model based on Higbie's penetration theory, which incorporates particle-to-interface adhesion, as well as a growing particle coverage during a liquid element's contact time at the interface, is used to analyze the experimental data. This model gives a reasonable description of the local auto-catalytic effects on the gas absorption rate.