The core light‐harvesting LH1 protein from Rhodospirillum rubrum can dissociate reversibly in the presence of n ‐octyl‐β‐ d ‐glucopyranoside into smaller subunit forms, exhibiting a dramatic blue‐shift in absorption. During this process, two main species are observed: a dimer that absorbs at 820 nm (B820) and a monomer absorbing at 777 nm (B777). In the presence of n ‐octyl‐β‐D‐glucopyranoside, we have previously shown that the B820 form is not only constituted by the αβ heterodimer alone, but that it exists in an equilibrium between the αβ heterodimer and β 2 homodimer states. We investigated the dissociation equilibrium for both oligomeric B820 forms. Using a theoritical model for αβ and β 2 , we conclude that the B820 homodimer is stabilized by both hydrophobic effects (entropy) and non‐covalent bonds (enthalpy). We discuss a possible interpretation of the energy changes.