Abstract The configuration at C-18 of variabilin (1), isolated from a Caribbean sponge, Ircinia felix, was established to be R. Variabilin acetate (2) was oxidatively degraded and converted into di-(R)-MTPA ester of 2-methylheptane-1,6-diol, which was found to have 2R -configuration by 1H-NMR studies in comparison with synthetic (2S, 6R S)- and (2R S, 6R S)-esters. The occurrence of (18R)-variabilin was demonstrated for the first time, whereas its (18S)-antipode were reported previously.