In the recent years, Raman and IR spectroscopies have attracted increasing attention as fast, non-invasive and widely applicable alternative analytical approaches for a variety of materials. Vibrational spectroscopy has been used in the analysis of herbal products, dyes and sensitive art objects, besides complex and aqueous biomaterials such as biopolymers or mammalian tissue. Compared to conventional analytical methods based on high-performance liquid chromatography (HPLC) or gas chromatography, which often involves extensive and time-consuming sample preparation, Raman or IR spectroscopy can avoid these procedures. The present work introduces a fast and reliable quantification method for the determination of naturally occurring indigo dye in dyer's knotweed (Polygonum tinctorium) based on Fourier transform (FT) Raman spectroscopy. The results were validated by HPLC-UV, and the merits and drawbacks of the present method are elaborated. Besides the qualitative aspects of signal assignment and comparison to appropriate attenuated total reflectance Fourier transform infrared (ATR-FT-IR) measurements, the Raman spectrum of dihydro indigo, an important intermediate in the indigo dying process, is presented for the first time and discussed with regard to its spectroscopic behaviour. Copyright © 2010 John Wiley & Sons, Ltd.