Solution phase NMR structures of carbamazepine dimers have been determined in CD3OH and in CDCl3. Dimerization is mediated by aromatic interactions in CD3OH and by H-bonding in CDCl3. Comparison of the two solution phase structures with dimer motifs found the X-ray crystal structure reveals remarkable similarities. The results suggest that the solution dimers represent first steps on the pathway to crystal nucleation and show that the structures of these prenucleation aggregates depend strongly on solvent.