To solve details of the self-assembling process of guanosine in diluted aqueous solution and to derive a thermodynamical model for quadruplex formation, the structural behavior of deoxyguanosine 5'-monophosphate has been analyzed by in-solution small angle X-ray scattering. The experiments have been performed as a function of guanosine concentration and at fixed guanosine concentration but in the presence of varying amounts of KCl. As a result, the self-assembling process, in terms of both aggregate particle fractions and aggregate length, has been observed to be strongly dependent on composition and largely affected by excess potassium ions in the solution. In particular, the different aggregate forms have been resolved and their concentration derived as a function of sample composition. In accordance with a hierarchical aggregation process, a nucleation and elongation mechanism has been used to derive the thermodynamical parameters for self-assembling. The results show that the annealing and fragmentation steps play an important role in the aggregation process.