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Instituto de Recursos Naturales y Agrobiología de Sevilla, CSIC, P.O. Box 1052, E- 41080 Seville, Spain E-mail address: delrio@irnase.csic.es (J.C. del Río), The chemical composition of leaf fibers of abaca (Musa textilis), which are commonly used for high-quality paper pulp production, was thoroughly studied. The results revealed that the lignin content was 13.2 % of the total fiber. The analysis of abaca fibers by Pyrolysis coupled to gas chromatography-mass spectrometry (Py-GC/MS) released predominantly compounds arising from lignin and p-hydroxycinnamic acids, with high amounts of 4-vinylphenol. The latter compound was demonstrated to arise from p-coumaric acid by pyrolysis of abaca fibers in the presence of tetramethylammonium hydroxide (TMAH), which released high amounts of p-coumaric acid (as the methyl derivative). Products from p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) propanoid units, with a predominance of the latter (H:G:S molar ratio of 1.5:1:4.9) were also released after Py-GC/MS of abaca fibers. Sinapyl and coniferyl acetates, which are thought to be lignin monomer precursors, were also found in abaca. The extractives content of the abaca fiber (0.4 %) was low, and the most predominant compounds were free sterols (24% of total extract) and fatty acids (24% of total extract). Additionally, significant amounts of steroid ketones (10%), triglycerides (6%), ω-hydroxyfatty acids (6%), monoglycerides (4%), fatty alcohols (4%) and a series of p-hydroxycinnamyl (p-coumaric and ferulic acids) esterified with long chain alcohols and ω-hydroxyfatty acids were also found, together with minor amounts of steroid hydrocarbons, diglycerides, α-hydroxyfatty acids, sterol esters and sterol glycosides., This study has been supported by the Spanish Ministerio de Ciencia y Tecnología (MCYT) (projects AGL2002-00393 and AGL2005-01748). We thank Isabel M. Rodríguez for sample preparation and technical support. We also thank J.M. Gras and G. Artal (CELESA, Spain) for providing the abaca leaf fibers.