The biosynthetic origins of the isocyanide and isothiocyanate functional groups in the marine sponge metabolites diisocyanoadociane (1), 9-isocyanopupukeanane (10) and 9-isothiocyanatopupukeanane (11) are probed by the use of [(14)C]-labelled precursor experiments. Incubation of the sponge Amphimedon terpenensis with [(14)C]-labelled thiocyanate resulted in radioactive diisocyanoadociane (1) in which the radiolabel is specifically associated with the isocyanide carbons. As expected, cyanide and thiocyanate were confirmed as precursors to the pupukeananes 10 and 11 in the sponge Axinyssa n.sp.; additionally these precursors labelled 2-thiocyanatoneopupukeanane (12) in this sponge. To probe whether isocyanide-isothiocyanate interconversions take place at the secondary metabolite level, the advanced precursor bisisothiocyanate 17 was supplied to A. terpenensis, but did not result in significant labelling in the natural product isocyanide 1. In contrast, in the sponge Axinyssa n.sp., feeding of [(14)C]-9-isocyanopupukeanane (10) resulted in isolation of radiolabelled 9-isothiocyanatopupukeanane 11, while the feeding of [(14)C]-11 resulted in labelled isocyanide 10. These results show conclusively that isocyanides and isothiocyanates are interconverted in the sponge Axinyssa n.sp., and confirm the central role that thiocyanate occupies in the terpene metabolism of this sponge.