Monosulfidic Black Ooze (MBO) is highly reactive and has a wide distribution in many Acid Sulfate Soil (ASS) drainage networks in northern New South Wales, Australia. There is little information on the chemical characteristics of these reactive sediments however; the mobility and subsequent oxidation of MBOs may pose a significant threat to water quality. In addition to their potential for deoxygenation and acidification of waterways, the acidic conditions found in drainage channels may result in the release of potentially toxic metals from both monosulfides (measured as Acid Volatile Sulfur (AVS) and disulfides (e.g. pyrite). Monosulfides are a reported indicator of metal toxicity in sediments with high AVS concentrations usually correlated with a high metal content. Characterized by their distinctive black color, MBO can be found as a reducing band, from only a few millimetres thick, to ooze layers over 1 m in depth. Their formation is thought to be controlled by the availability of iron (Fe), sulfate (SO₄) and Dissolved Organic Carbon (DOC) in the pore waters. This study was undertaken to determine limiting factors to monosulfide formation in ASS drainage systems. Porewater analysis indicated that none of the usual limiting factors (e.g. sulfate, organic matter, reactive iron) were limiting the formation of iron sulfide minerals in the MBO. The accumulation pattern of iron sulfides was representative of a young formation and deposition environment. Given the rapid accumulation of drain oozes, and the abundance of all the factors required for iron sulfide formation, the kinetics of the reactions must be the limiting factors. Whilst extremely high AVS concentrations were found (up to 6.16% S_AV), the metal concentrations found were generally within current guidelines. The correlation between high AVS concentration and high metal content found in previous studies, was not evident here.