This invention relates to compositions comprising co-crystals of (−)-gossypol with a C1-8 carboxylic acid or C1-8 sulfonic acid which are useful as inhibitors of Bcl-2 family proteins. The invention also relates to the use of co-crystals of (−)-gossypol with a C1-8 carboxylic acid or C1-8 sulfonic acid for inducing apoptosis in cells and for sensitizing cells to the induction of apoptotic cell death.