Self-organization of molecules in solution is an important natural and synthetic process, in particular for the preparation of nanomaterials. However, the mechanism of growth for solution-based nanoparticle formation is not always well understood. We present results that clarify these mechanisms in solutions of magnesium phthalocyanine in which the self-organization is induced by addition of a miscible nonsolvent. From simultaneous measurements of the sizes of the growing nanoparticles by photon correlation spectroscopy and the molecular concentration by absorption and fluorescence spectroscopy, we have found that the particles do not grow by molecular diffusion to the surfaces. These results suggest the importance of unstable clusters in the growth process. We also observed a strong dependence of the particle size on the initial concentration which we attribute to effects of the curvature of the solubility curve.