1,4a,7-Trimethyl-7-vinyl-1,2,3,4,4a,4b,5,6,7,8,10,10a-dodeca-hydro-phenanthrene-1-carboxylic acid.

The title compound, also known as isopimaric acid, C(20)H(30)O(2), was isolated from slash pine rosin. There are two unique mol-ecules in the unit cell. The two cyclo-hexane rings have classical chair conformations. The cyclo-hexene ring represents a semi-chair. The mol-ecular conformation is stabilized by weak intra-molecular C-H⋯O hydrogen-bonding inter-actions. The mol-ecules are dimerized through their carboxyl groups by O-H⋯O hydrogen bonds, forming R(2) (2)(8) rings.