The dependence of the optical properties of spherical gold nanoparticles on particle size and wavelength were analyzed theoretically using multipole scattering theory, where the complex refractive index of gold was corrected for the effect of a reduced mean free path of the conduction electrons in small particles. To compare these theoretical results to experimental data, gold nanoparticles in the size range of 5 to 100 nm were synthesized and characterized with TEM and UV-vis.