Distinctly different rates and pathways were observed for abiotic transformation of gamma-hexachlorocyclohexane (gamma-HCH) between homogeneous systems and systems containing FeS solid. The observed half-lives of gamma-HCH decrease from about 1136 and 126 d in homogeneous systems to about 55 and 50 d in heterogeneous systems at pH approximately 6.9 and pH approximately 8.3, respectively. While no appreciable difference in the measured rate was found between the two tested heterogeneous systems of pH 6.9 and pH 8.3, the observed hydrolysis rate constant for the homogeneous systems is highly pH dependent, being constant at approximately 0.0005 d(-1) in pH 2.26-6.73, and increased by an order of magnitude per pH unit from pH 6.73 to pH 12.02. A dehydrohalogenation pathway from gamma-HCH to pentachlorocyclohexene followed by parallel reactions to three trichlorobenzenes is proposed for the hydrolysis reaction in the homogeneous systems. For the heterogeneous systems, the reactions likely follow both the hydrolysis pathway and pathways from gamma-HCH to tetrachlorocyclohexene (TeCCH) and to dichlorocyclohexadiene (DCCD) via dichloroelimination followed by dehydrohalogenation from TeCCH to dichlorobenzenes or from DCCD to chlorobenzene. This study suggests that environmental modeling using hydrolysis rate data measured in homogeneous systems may bear significant inaccuracy for the predicted fate and transport of HCH isomers in highly heterogeneous ecosystems at the regional or mega scales.