Abstract: The object of the present invention is a novel mandelate salt of a substituted tetracyclic tetrahydrofuran derivative according to Formula (I) the N-oxide forms and the stereochemically isomeric forms thereof, wherein R1 and R2 each independently are hydrogen or C1-6alkyl and R3 and R4 each independently are hydrogen or halogen. In the foregoing definitions C1-6alkyl defines straight and branched saturated hydrocarbon radicals having from 1 to 6 carbon atoms such as, for example, methyl, ethyl, propyl, butyl, 1-methylpropyl, 1,1-dimethylethyl, pentyl or hexyl. The novel mandelate salt is not light-sensitive and is far more stable than the prior art salts at room temperature, enhanced temperature and at relative high humidities and in aqueous media.