We have performed small-angle X-ray scattering with a synchrotron source on dilute suspensions of colloidal spheres of polystyrene latex, Stöber silica, and microemulsion-grown silica. Many interference fringes are observed of the monodisperse particles over a large range of scattering vectors and more than 5 orders of magnitude in intensity. We present a straightforward method to deduce the radii, the size polydispersity, and the interface thickness of the particles from a Porod plot of one and the same in situ measurement. The radii agree very well with static light-scattering data. The radii are larger than the electron microscopy data of dry spheres and smaller than the hydrodynamic radii from dynamic light-scattering data. The size polydispersities are smaller than those obtained by electron microscopy, which is well explained by the intrinsic random errors of electron microscopy. We find that nearly all the particles have a homogeneous internal density and a sharp interface with the suspending medium of less than 1 nm wide. In one case of a stepwise synthesized particle, we have discerned a dense core and a less-dense shell, without contrast matching with the suspending liquid. It is concluded that synchrotron small-angle X-ray scattering is a very powerful technique for the in situ study of colloidal systems.