We investigated the differences between the decanethiolate gold nanoparticles synthesized by two different routes: one-phase and two-phase methods. Their properties were compared in bulk and at the air-water interface by transmission electron microscopy (TEM), X-ray reflectivity (XR), extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray powder diffraction (XRD), thermal gravimetric analysis (TGA), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), electron paramagnetic resonance (EPR), and Langmuir-Blodgett technique. The mean nanoparticles sizes obtained by EXAFS and XRD were found to be smaller than those by the TEM measurements. We explained these differences by the structural disorder and multiple twinning in the nanoparticles. The one-phase particles were found by EXAFS to be smaller and had a higher grafting density of thiol chains than the two-phase particles. We attributed these differences to the enhanced disorder of the one-phase particles. At the air-water interface, the one-phase particles did not spread, while the two-phase particles spread and formed Langmuir films. TEM and XR results revealed that the close-packed monolayer of the two-phase particles collapsed and folded into multilayer films upon further compression.