In the framework of development of new processes for spent nuclear fuel reprocessing, new extractant molecules are studied. The goals of this thesis are to study the molecular and supramolecular speciation of representative organic solutions. The speciation was determined by coupling experiments and theoretical tools. After solute extraction, the composition of the organic solutions is experimentally determined. Simulations boxes with the same composition than experimental solutions are build. After simulation, trajectories are used to calculate small angle scattered intensities. The representativeness of the simulations is checked by comparison of experimental and calculated scattered intensities. The use of the simulation, ESI-MS spectrometry and IR spectroscopy provides the description of the structures in organic solution at the molecular and supramolecular scale. This methodology was applied to water and uranyl nitrate extraction by the monoamide DEHBA and MOEHA and by the malonamide DMDOHEMA as well as the extraction of water and neodymium nitrate by TODGA solutions.The extraction of water depends on of the organization of the solutions: monoamide solutions composed of monomer and dimer solubilize few amounts of water in comparison with DMDOHEMA or TODGA solutions. The extraction of uranyl nitrate showed several behaviors. UO2(NO3)2L2 complexes are observed at low uranium concentration whereas polymetallic species are observed when the uranium concentration increases. In the case of DMDOHEMA, monometallic complexes are major species. The uranyl can be linked to 1 o 2 malonamides, 2 nitrates and sometimes to a water molecule. Neodymium is extracted by TODGA within small aggregates composed of 2 or 3 cations linked by bridging nitrates. The increase of the concentration of neodymium leads to a phase separation. The presence of octanol allows the extraction of higher concentration of neodymium nitrate without splitting of the organic phase. Simulations showed the replacement of water and TODGA molecules in the first coordination sphere of the cation with octanol molecules. This increases the solubility of the aggregates. The presence of DMDOHEMA prevents also the phase separation by structuring the solution. With malonamide, smaller aggregates are observed. For both systems, phases after splitting (third phase formation) were characterized. This work provides the study of the molecular and supramolecular organization of organic solutions by combining experiments and molecular dynamic simulations.