Fractionation of the CH2Cl2 phase of the MeOH extract from the leaves of Calea pinnatifida (Asteraceae) led to the isolation of two related sesquiterpene lactones (STLs): calein C (1) and calealactone B (2). Additionally, during the purification process a derivative of calein C (3) was formed as product of Michael addition of MeOH. The structures of compounds were established based on spectroscopic and spectrometric data [1], while the absolute stereochemistry was established by vibrational circular dichroism for the first time in the literature. VCD associated with density functional theory (DFT) calculations has been demonstrated as a powerful probe to chirality in natural products [2]. The experimental spectra obtained in CDCl3 and simulated data at the B3PW91/6-311G(d,p) level led to the assignment of 4R,6R,7S,8S,9R,10R to compound 1. The most representative vibrational modes used in this assignment were those at (-)-1025 cm-1 (C-O stretch of C-6, C-8, C-9 and C-10), (-)-1125 and (-)-1145 cm-1 (C-H bending of whole molecular framework), as well as (-)-1345 and (-)-1365 cm-1 (out-of-plane and in-plane C-H bending of C-6, C-7, C-8 and C-9). On the other hand, NMR and MS data analysis of (2) demonstrated that this STL is an analogue of (1), containing an epoxide ring at C-2 and C-3. Comparisons of experimental and calculated spectra allowed the assignment of (-)-2 as 2S,3S,4R,6R,7S,8S,9R,10R. As expected, the VCD spectra obtained for (-)-2 were very similar to those of (-)-1 as they share most of the structural and stereochemical features. This is the first report in the literature of the absolute configuration of these sesquiterpene lactones