The title compound, [systematic name: 5a-acet­oxy­methyl-3-isopropyl-8-methyl-1,2,3,3a,4,5,5a,6,7,10,10a,10b-dodeca­hydro-7,10-endo-epidi­oxy­cyclo­hepta­[e]indene-3a-carb­oxy­lic acid], C22H32O6 (I), is closely related to methyl 5a-acet­oxy­methyl-3-isopropyl-8-methyl-1,2,3,3a,4,5,5a,6,7,10,10a,10b-dodeca­hydro-7,10-endo-epidi­oxy­cyclo­hepta­[e]indene-3a-carboxyl­ate, (II) [Brito et al., (2008 ▶). Acta Cryst. E64, o1209]. There are two mol­ecules in the asymmetric unit, which are linked by two strong intra­molecular O—H⋯O hydrogen bonds with graph-set motif R 2 2(8). In both (I) and (II), the conformation of the three fused rings are almost identical. The five-membered ring has an envelope conformation, the six-membered ring has a chair conformation and the seven-membered ring has a boat conformation. The most obvious differences between the two compounds is the observed disorder of the acet­oxy­methyl fragments in both mol­ecules of the asymmetric unit of (I). This disorder is not observed in (II). The crystal structure and the molecular conformation is stabilized by intermolecular C—H⋯O hydrogen bonds. The ability to form hydrogen bonds is different in the two compounds. The crystal studied was a non-merohedral twin, the ratio of the twin components being 0.28 (1):0.72 (1)