The analysis of indigoid dyes as leuco forms by NMR spectroscopy*

1 H and 13 C NMR spectra of halogen‐ and alkoxy‐substituted indigoid dyes (indigotins), and of some unknown deuterated indigotins as leuco forms (i.e., reduced forms or ‘vats’), in deuterium oxide are recorded and discussed. NMR spectroscopy is a useful technique for the identification and characterisation of this traditional class of vat dyes.