Chemically synthesized nanoparticles can be regarded as building blocks for more complex systems. Hence, the characterization of their attributes is a key step toward controlling and tuning the properties of new nanocomposite materials. We present a structural characterization of thiol-capped Sold nanoparticles as a function of size (2-4 nm) by X-ray diffraction (XRD). In addition to the fee bulk structure, nanometric gold may present decahedral or icosahedral structures depending on particle size. To evaluate the structure evolution, we have applied Debye function analysis to the XRD profiles. The analysis reveals a high proportion of defective and complex structures in all samples, which may have become trapped by the growth conditions Nevertheless, the fee structure is more prominent with increasing diameter, indicating that thermodynamics also plays an important role in the average structure of this system. The results underline the importance of performing careful structural characterization of chemically synthesized nanoparticles, in particular, in systems such as gold, where several distinct and stable structures are known to have little difference in energy.