Three sesquiterpene lactones were isolated from the aerial parts of Centaurea sinaica: amberboin, chlorohyssopifolin A and sinaicin. The latter is reported for the first time. Centaurea species (Compositae, Cynareae) contain a number of sesquiterpene lactone structural types, [1-3], especially guaianolides [4-8], many of which possess interesting biological properties [9]. In this paper we report the isolation and structure elucidation of three guaianolides from the aerial parts of C. sinaica DC. which grows in Qatar, and has not been previously investigated. Two of these lactones were already known, and were identified as amberboin, C₁₅H₂₀O₄ (1) [10, 11], and chlorohyssopifolin A (centaurepensin), C₁₅H₁₇O₄Cl₂, (2) [12]. The third compound, (3, R=Ac) is new and has been given the name sinaicin.