The molecular structures of “isobishydropulegone” and “Harries–Roeder ketol,” isomers of “bishydropulegone,” which are dimeric reduction products of pulegone, were determined as being (1S, 3′aR, 4R, 6′R, 7′aS)-7′a-hydroxy-2′,2′,3′,3′,4,6′-hexamethylspiro[cyclohexane-1,1′-octahydroinden]-2-one and (1R, 3′aS, 4R, 6′R, 7′aS)-7′a-hydroxy-2′,2′,3′,3′,4,6′-hexamethylspiro[cyclohexane-1,1′-octahydroinden]-2-one respectively by the analysis of the two-dimentional NMR spectra.