Yellow Cross (Gelbkreuz) is a World War I chemical warfare agent usually based on mustard gas (sulfur mustard, HS, Yperite, Lost).
The original Gelbkreuz was a composition of 80–90% of sulfur mustard and 10–20% of tetrachloromethane or chlorobenzene as a solvent which lowered its viscosity and acted as an antifreeze, or, alternatively, 80% sulfur mustard, 10% bis(chloromethyl) ether, and 10% tetrachloromethane.
A later formulation, Gelbkreuz 1, was a mixture of 40% ethyldichloroarsine, 40% ethyldibromoarsine, and 20% of bis(chloromethyl) ether.
French "ypérite no.20" was a similar mixture of 80% sulfur mustard and 20% tetrachloromethane.
Yellow Cross is also a generic World War I German marking for artillery shells with chemical payload affecting the skin.