SiCN fibers with a mean diameter of 50 nm and an aspect ratio of up to 100 are produced in a two-step process by R. Kempe and co-workers. The key step to fabricate the longitudinal and crosssectional views of the mesofibers shown here is a concerted block-copolymer synthesis, microphase separation, and cross linking at 140 °C followed by pyrolysis at 1100 °C. Inexpensive components like a commercially available silazane and polyethylene are linked. The fibers may find application in electronic devices, as components of ceramic matrix composites, as fiber beds in hightemperature nano-filtering like diesel fine dust removal, or as thermally robust and chemically inert catalyst supports. Furthermore, the SiCN nanofibers introduced on page 984 are a promising alternative to ultrathin carbon fibers, due to their oxidation resistance.