An unusually large zero-field splitting of the lowest triplet state has been observed directly in the phosphorescence spectrum of the xanthione molecule in a xanthone host crystal. There are two xanthione guest sites in the host crystal. In one of the sites the zero-field parameter D is -11 cm-1 and in the other it is -20 cm-1. These results have been confirmed by measurements of the Zeeman splittings. It is argued that the large D values are due to spin-orbit coupling. © 1980.