The composition of the indigoid dyes and their precursors derived from the hypobranchial glandular secretions of Murex trunculus and Murex brandaris have been determined by high-resolution mass spectrometry. M trunculus secretions yielded three dyes: 6,6'-dibromoindigotin (DBI; Cl 75800), indigotin (Cl Natural Blue 1; Cl 75780) and 6-bromoindigotin (MBI), the latter reported here for the first time. Indirubin (Cl Natural Blue 1; Cl 75790) constituted a relatively small amount of the final dye product. No statistically significant sexual differentiation in the final dye colorations was observed when the secretions were fully developed in light. When developed in the dark, male glandular secretions were more purple than those of pseudohermaphroditic females, implying that male glands contain less of the unsubstituted indoxyl precursors, which oxidatively couple in air to form blue indigotin and MBI. M brandaris yielded solely DBI and its precursor the 6-bromo-2-methylthioindoxyl (tyrindoxyl); no methylsulphone precursor was observed, although reported by earlier investigators. Moreover, exposure to light was found to be critical for the formation of the greenish-coloured intermediate 2,2'-bis-substituted 6,6'-dibromo-2,2'-di-indoxyl, not only for the latter's decomposition to DBI.