Hitherto sulfur flows have been recognized as lobate features similar in form to basaltic lava flows. However, we have discovered a self-combusting sulfur-flow mode that leaves an entirely different and unexpected deposit. In this mode, the flow is emplaced in a combusting state, so that all sulfur is burned away to leave a sulfur-free, thermally eroded trough. During the 4-hr-long event we observed, combustion of 0.6 m3 of sulfur generated 2.4 tons of SO2. Once under way, combusting flows do not require eruption of a molten volume to maintain activity: They can generate supply volume by melting surficial sulfur along the flow path. Combusted flow features are widespread at Vulcano, Italy, indicating that this previously unknown emplacement mode may be common. Previous failure to recognize this flow style may account for the apparent rarity of sulfur flows. Our new findings overturn conventional thought on how sulfur flows are emplaced, interpreted, and considered, and may show them to be a common volcanic feature.