Gas chromatography and mass spectrometry of 20 samples of Thysananthus (T. comosus, T. convolutus, T. retusus, T. spathulistipus) and one of Dendrolejeunea (D. fruticosa) from Malaysia and Thailand revealed the presence in most taxa of large quantities of sesquiterpenoids and unidentified diterpenoids. Isolepidozene was detected as a major component in T. comosus, T. convolutus and T. spathulistipus, pinguisane sesquiterpenoids as major components in T. retusus, and the unidentified (M+)304, 271(100), 105(90) as the most abundant component of D. fruticosa. The large chemical heterogeneity detected in T. convolutus correlates with the variation in leaf dentation observed in the species and supports the resurrection of T. gottschei, previously considered a synonym of T. convolutus. The subdivision of Thysananthus into two subgenera, subg. Thysananthus and subg. Sandeanthus, and the treatment of Dendrolejeunea as a separate genus, are chemically supported.