The availability of many diterpenes of the (-)-kaurene series and of beyerol in local species of the Euphorbiaceae has prompted us to attempt interconversion of these skeletons. The first suggestion that such a rearrangement might occur is implicit in Wenkert's formulation of mirenel based on its reported acid isomerization to isophyllocladene. Although mirene is now known to be a mixture containing isophyllocladene, the structure has been recognized in beyerene (Ia), and Wenkert's proposal includes the reversible isomerization of beyerene (Ia) and (-)-kaur-15-ene (IIa). Kaurene itself (IIIa) undergoes partial isomerization to isokaurene (IIa), when treated with acid under a variety of conditions, although no skeletal rearrangement has been reported. 3a,19-Ethylidenedioxybeyer-15-ene (Ib), available from (IV) by tosylation followed by reduction with lithium aluminium hydride, was resistant to similar acid treatment and under more severe conditions gave a complex mixture. It was anticipated that the more selective approach of solvolysing the 16-exo tosylate (VIg) should lead to rearrangement, since stereoelectronic factors would favour participation of the C 12-C 13 bonding electrons. In an earlier paper we described briefly the introduction of oxygen into position 15 and 16 of the beyerene molecule by hydroboration of (IV). The epoxide derived from (IV) also gave the 15β,17-diol after reduction with lithium aluminium hydride. A distinction between the glycols was made by intramolecular hydrogen bonding which is not possible for either of the 15,17-diols.