This study represents the first comprehensive investigation into the use of stable sulfur isotope ratios in Acid Sulfate Soils (ASS). Samples representing both contemporary and ancient sulfide formations were collected from a variety of ASS including coastal and inland sediments and monosulfidic black ooze. The isotopic composition of the acid volatile sulfur, chromium reducible sulfur and soluble sulfate fractions were determined. Each of the fractions recorded a wide variation in δ³⁴S values. The AVS fraction displayed δ³⁴S values that ranged between -19.0‰ and +16.8‰, while the CRS fraction ranged from -28.6‰ to +14.4‰. δ³⁴S of the soluble sulfate fraction varied between -16.4‰ and +29.1‰. In many of the ASS examined, similarities between the δ³⁴S of the sulfide and sulfate fractions indicated the primary source of sulfate was derived from the oxidation of sulfides. At one coastal site, this process was contributing to the isotopic composition of the sulfate in the fresh water overlying the site. This resulted in the sulfate having an isotopic signature well outside the range previously given for freshwater sulfate. In future studies this research may be used to determine the contribution the oxidation of ASS is making to the sulfate concentration in surface and groundwaters or to identify sites of sulfide oxidation. In geological and sedimentary studies, fractionation of the sulfide is calculated from a known seawater sulfate value (~20.6‰). At many of the sites examined in this study however, the degree of seawater influence was limited, non-existent or unknown and sulfate was supplied from other sources. For this reason, fractionation was also calculated from the soluble sulfate, as this is likely to be more indicative of the precursor sulfate source. The results indicated that applying a known seawater sulfate value led to the degree of fractionation being overestimation on many of the coastal sites and underestimated on some of the inland sites. At sites where the influence of seawater was identified, the δ³⁴S of the soluble sulfate was still found to be below the known seawater sulfate value due to the effect of bioturbation and tidal activity. For these reasons, it was concluded that the isotopic composition of the soluble sulfate should, where possible, be determined and that this value be used to calculate the degree of fractionation in preference to applying a known seawater sulfate value. The classification of a system as open or closed with respect to sulfate supply adopted for many geological and sedimentary studies is based mainly on seawater being the precursor sulfate source. Given contemporary ASS varied in their source of precursor sulfate, this classification system was found to be unsuitable and a new system proposed. The proposed classification system is based on the fractionation between the soluble sulfate (considered to be more indicative of the precursor sulfate source) and the sulfide and recognizes the dynamic nature of many ASS environments. An important feature of the new classification system is that it allows the classification of non-seawater influenced landscapes. Given the procedure to extract sulfides and sulfates for isotope analysis is relatively simplistic, the cost of the analysis is comparable to other standard laboratory tests and the quality of the information received, it was concluded that sulfur isotope ratios are a valuable tool to help understand environmental processes occurring in ASS landscapes.