Tri-t-butylgallium has been reacted with the macrocycle 1,12-diaza-3,4: 9,10-dibenzo- 5,8-dioxocyclopentadecane which could be a potential ligand for gallium(III). A reaction product was analyzed and single crystal X-ray diffraction experiments showed that it consisted of the cyclic dimer of di-t-butylgallium-hydroxide hydrogen bonded to the macrocycle. Without a co-crystallized organic molecule di-t-butylgallium hydroxide crystallizes as a trimer. Density functional calculations have been used to predict the structures and the total energies for the monomer, dimer, trimer, and tetramer of dimethylgallium hydroxide in order to provide a basis for the understanding of oligomer population for the dimethylgallium hydroxides. Force field calculations are shown to be able to produce a similar strain energy difference for dimer, trimer, and tetramer forms of (CH3)(2)Ga(OH) and this method can economically be used for larger alkyl groups. The force field computations show that the trimeric di-t-butyl gallium( III) hydroxide is much more stable than the dimeric form which therefore must owe its existence to the association with the hydrogen-bonded macrocycle.