The structural investigation of four bio- and synthetic melanins, obtained by the action of polyphenol oxidase from potatoes and Psalliota campestris mushrooms and of tyrosinase from Sepia officinalis on tryptophan, or by means of performic oxidation, has been carried out by thermal decomposition performed in the ion source of a mass spectrometer. The structural characterization of the ionic species [C8H7ON](+.), common and abundant for all the examined compounds, has been obtained by accurate mass measurements and collisional spectroscopy. These mass spectrometric techniques have shown unequivocally for ionic species [C8H7ON](+.) the structure of 2-indolinone.

The electron impact mass spectrometric behaviour of 2‐(methoxycarbonylmethyl)‐2‐phenyl‐1,3‐benzoxa‐thiole 3a and 2‐acetonyl‐2‐phenyl‐1,3‐benzoxathiole 3b has been studied in detail. The structures of the more interesting fragment ions have been confirmed with the aid of accurate mass measurements, linked scans and collisional activation experiments. In addition to the ionic species already described in the general fragmentation pattern of 1,3‐benzoxathiole derivatives, other fragments arising from the interaction of the carboxy‐containing chain with the sulfur atom of these compounds have been observed and evidence presented.