Two dibenzylbutane lignans, dihydroguairetic acid and austrobailigan-6 were isolated as mixture of their diastereoisomers from the bark ethanolic extract of Dugandiodendron argyrotrichum (Magnoliaceae), a colombian endangered species. Characterization was achieved by 1H and 13C NMR, DEPT, COSY, HMQC, HMBC and GCMS analysis. The diastereoisomer separation by GC and the diastereospecific fragmentation patterns in negative chemical ionization mass spectrometry (NCI-MS) spotlights on an interesting tool for stereochemical research.