The most intriguing feature of the plot in Figure is the ca. 4h induction period before H2 is detected -behavior that is often linked with catalyst evolution toward a more potent form. Following closed-cell sample irradiation, the opened photochemical cell emitted an H2S-like odor. Qualitative assessment of the solution in the cell using a commercial silver-containing test strip yielded a change in strip color from white to dark brown, consistent with silver sulfide formation. XPS examination of the darkened test strip confirmed the presence of S 2-, but neither cobalt nor zirconium. Finally, post-catalysis assessment of PTM@MOF composition revealed losses of between 3.5 and 6 sulfide ions from the ca. 24 initially present. The combined results point to sulfide loss as a key step in converting CoMo6S24@NU1K from catalytically inactive to catalytically active form.