The environmental expression of sulfur compound disproportionation has been placed between 640 and 1050 million years ago (Ma) and linked to increases in atmospheric oxygen. These arguments have their basis in temporal changes in the magnitude of 34 S/ 32 S fractionations between sulfate and sulfide. Here, we present a Proterozoic seawater sulfate isotope record that includes the less abundant sulfur isotope 33 S. These measurements imply that sulfur compound disproportionation was an active part of the sulfur cycle by 1300 Ma and that progressive Earth surface oxygenation may have characterized the Mesoproterozoic.