Here we report a mechanistic study on the diversity of assembled structures. We take the assembly of porphyrin molecules as an example to investigate how the diffusion of molecules influences the structure of assemblies. The diffusion is regulated by changing the composition of solvents. Diverse structures of porphyrin particles are assembled at various volume ratios of ethanol in water, including strip-like particles forming at 50%, tubular particles forming at 80% and rod-like particles forming at 95%. The in situ examination on the self-assembly process by confocal microscopy reveals that at the low volume ratio the nucleation initiates at the interface of emulsion followed by the growth of particles in the solvent. The diffusion limitation of molecules in the solvent is remarkable at the low volume ratio of ethanol as a result of high viscosity of the solvent and low solubility to the molecule, which enables the anisotropic growth of particles forming strip-like particles. With the increase of volume ratio of ethanol, the diffusion limitation is attenuated, leading to the thickening of particles and the formation of rod-like products. The thickening under diffusion limitation also causes a preferential growth of outer layer of particles resulting in the formation of hollow structures. The findings in this paper show that the diffusion of molecules plays an important role in shaping structures of assemblies, which was underestimated in previous studies. (C) 2015 Elsevier B.V. All rights reserved.