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As part of our continuing studies of lactones from the aerial part of the plant Rhaponticum serratuloides, 15-O-deacetylrhaposerin and new guaianolide called rhaserin were isolated along with the well-known loliolide. The configuration of the C(17) asymmetric center in 15-O-deacetylrhaposerin was established by X-ray diffraction analysis. The molecular structure of rhaserin was determined based on the data from 1H and 13C NMR spectroscopy (including 2D NMR spectroscopy) of this lactone and its 4,15-O-isopropylidene derivative.