High resolution mass spectrometry was used to identify indigoid dyes of molluscan origin. Indigoids on modern and ancient textile fibers as well as indigoid mixtures produced by reactions are discussed. Steam treatment of the fiber or isolation of the dye by solvent extraction permits the analysis of dyes that bind more tightly to fibers such as 6,6'-dibromoindigotin (C.I. 75800, Natural Vat Dye). The demonstration of the stepwise photolytic debromination of the leuco base of 6,6'-dibromoindigotin may be of more far-reaching significance in distinguishing between natural and intentional mixtures of indigotin and its dibromo derivative.