Significant variability in ?34Spyrite values in Neoproterozoic sedimentary rocks has been attributed to the evolution of nonphotosynthetic sulfide-oxidizing bacteria and the advent of sulfur disproportionation reactions in response to Earth's evolving redox chemistry. We analyzed trace sulfate in carbonates from South Australia and Namibia and reconstructed the sulfur isotope evolution of seawater sulfate. Comparison of our ?34Ssulfate record with published ?34Spyrite data from the same or equivalent successions indicates that ?34Ssulfate - ?34Spyrite (?34S) rose gradually through the second half of the Neoproterozoic and fluctuated coincident with episodes of glaciation, but did not exceed 46? before ca. 580 Ma. Large variability in ?34Spyrite in the Neoproterozoic can be explained as a consequence of low sulfate concentrations and rapidly fluctuating ?34Ssulfate in seawater rather than the onset of sulfur disproportionation reactions mediated by nonphotosynthetic sulfide-oxidizing bacteria.