Measurements of the elastic constants, the specific heat and the entropy of grain boundaries have been performed by utilizing ultra-fine grained materials (Pd, 6nm grain size; Mg, 12 nm grain size; CaF 2, 7 nm grain size) about 30 to 50 vol% of which consist of grain boundaries. The results obtained suggest that (1) The interatomic potentials in grain boundaries in metals differ from the lattice potentials. Hence, the application of lattice potentials for computer simulation of boundary structures in metals may be of limited physical relevance.(2) Boundary entropy effects are significant for the interfacial structure above half the absolute melting temperature.(3) Metastable boundary structures with enhanced boundary entropies exist. The entropy enhancement may be several times the entropy of an equilibrated interface in the same material.