Significant variability in delta(34)S(pyrite) 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 delta(34)S(sulfate) record with published delta(34)S(pyrite) data from the same or equivalent successions indicates that delta(34)S(sulfate) - delta(34)S(pyrite) (Delta(34)S) rose gradually through the second half of the Neoproterozoic and fluctuated coincident with episodes of glaciation, but did not exceed 46parts per thousand before ca. 580 Ma. Large variability in delta(34)S(pyrite) in the Neoproterozoic can be explained as a consequence of low sulfate concentrations and rapidly fluctuating delta(34)S(sulfate) in seawater rather than the onset of sulfur disproportionation reactions mediated by nonphotosynthetic sulfide-oxidizing bacteria.