Deprotonation of sulfonium and oxosulfonium salts affords ylides, of structure RS−C−R′ and RS(O)−C−R′. While sulfonium ylides, for instance in the Johnson–Corey–Chaykovsky reaction used to synthesize epoxides, are sometimes drawn with a C=S double bond, e.g., RS=CR′, the ylidic carbon–sulfur bond is highly polarized and is better described as being ionic. Sulfonium ylides are key intermediates in the synthetically useful Stevens rearrangement. Thiocarbonyl ylides (RR′C=S−C−RR′) can form by ring-opening of thiiranes, photocyclization of aryl vinyl sulfides, as well as by other processes.