A technique for determination of the size of particles in solution. The intensity of scattered X-rays from irradiation of a particle, e.g. a ribosome or a virus, is related to its size. In a variant technique, the solution of particles is irradiated with neutrons, the scattering of which varies with the mass of an atom's nucleus and thus can indicate the size of a phosphorus (i.e. nucleic acid)-containing particle or a ²H-labelled protein.