The temperature dependences of the coefficients representing preferential grain-boundary diffusion (PGBD), the dependences of the PGBD parameters on excess charges of ``atomic probes,'' and the isotopic effect under the PGBD conditions made it possible to identify the interstitial PGBD mechanism for a number of f.c.c. metals. A new effect common to PGBD in metals, has been found: a kink appears in the temperature dependences of the PGBD coefficients. A novel method utilizing atomic probes which give the spectral information has provided data demonstrating a lower density of conduction electrons and lower dynamic parameters of the cores (interiors) of grain boundaries, has proved the existence of atomic probes in the form of mixed split interstitials in grain boundaries, has provided data on the properties of an adsorption layer adjoining a grain boundary (which is responsible for the kink in the temperature dependences of the PGBD coefficients), and has yielded the characteristics of magnetic phase transitions occurring in two-dimensional regions in the core of a grain boundary and in adjoining layers of the lattice of thickness representing several interatomic distances.