The diffraction experiment provides explicit data which relate directly to the electron density (X-ray scattering) or the magnetization density (polarized neutron scattering). A summary is given of the determination of spin densities in a number of paramagnetic molecular ions with orbitally non-degenerate ground states. The observed densities are compared with theoretical models up to the level of unrestricted Hartree-Fock. Electron correlation is amongst the most important features in the molecular spin densities; observed spin transfer from the metals to the ligands can be matched only with a theory based on a quite extensive basis set.