Young fustic (Cotinus coggygria Scop.; Anacardiaceae) has been used as a dyestuff since antiquity. Phytochemical investigation of the methanol extract of the heartwood has led to the isolation and structure elucidation by nuclear magnetic resonance and mass spectrometry (MS) of 3',4',6-trihydroxyaurone (sulfuretin) and 3',4',7-trihydroxyflavonol (fisetin) as well as 3',4',7-trihydroxyflavanol (fustin), 3',4',5,7-tetrahydroxyflavonol (quercetin), 3',4',5,7-tetrahydroxyflavanol (taxifolin), 4',7-dihydroxyflavanol, 3',4',7-trihydroxyflavanone (butin), 4',7-dihydroxyflavanone (liquiritigenin), trans-2',3,4,4'-tetrahydroxychalcone (butein), 4',5,7-trihydroxyflavanone and trans-2',4,4'-trihydroxychalcone (isoliquiritigenin). The isolated compounds were used as reference materials for the development of a high-performance liquid chromatography-diode array detector-MS method, which was then applied to analyse (1) fresh silk samples dyed with young fustic, (2) dyed silk subjected to artificially accelerated light ageing and (3) historical silk micro-samples, extracted from ecclesiastical post-Byzantine garments (fifteenth to eighteenth century), which belong to monasteries of Mount Athos. Sulfuretin and fisetin, which are usually used as markers for the identification of the yellow dye and, for the first time, some of the aforementioned flavonoid components of young fustic were identified in the historical extracts. Furthermore, preliminary experiments suggested that although the amounts of the dye components decrease with light ageing, the relative ratio of fisetin and sulfuretin, after a first step of ageing, seems to be almost unaffected by such degradation processes raised by light. The effect of the latter on the morphology of the dyed silk fibres is briefly investigated by scanning electron microscopy.