The assemblage grandite garnet plus epidote is observed in contact metasomatic deposits in Japan, such as those of the Chichibu, Kamaishi, Yaguki, Nakatatsu, and Fujigatani mines. The composition and compositional zoning of minerals, principally from the Chichibu mine, were analyzed by means of the electron probe. The results indicate that garnet and epidote coexist with each other over wide composition ranges and that the garnet can be approximately represented by the binary system andradite (Ca 3 Fe 2 Si 3 O 12 )-grossular (Ca 3 Al 2 Si 3 O 12 ), and epidote by the binary pistacite (Ca 2 Al 2 FeSi 3 O 12 (OH))-clinozoisite (Ca 2 Al 2 AlSi 3 O 12 (OH)).Al-Fe partitioning between coexisting garnet and epidote is measured at their grain boundaries, assuming surface equilibrium was maintained in the exchange reaction expressed as follows:Ca (sub 3/2) FeSi (sub 3/2) O 6 + Ca 2 Al 2 AlSi 3 O 12 (OH) = Ca (sub 3/2) AlSi (sub 3/2) O 6 + Ca 2 Al 2 FeSi 3 O 12 (OH))andradite clinozoisite grossular pistaciteBy plotting on a partition diagram with axes of logarithm of Al/Fe atomic ratios in garnet and in the M(3) site of epidote, the following results were obtained:(1) Garnet and epidote solid solutions can be approximated as ideal mixtures in the binary systems.(2) Natural assemblages plot in a limited area on the partition diagram, and the partition coefficient (K' = (Al/Fe) in garnet/(Al/Fe) in the M(3) site of epidote) is generally in a range from 1.7 to 10.(3) The partition coefficient depends mainly on temperature. Mineral pairs formed under relatively high temperatures have small partition coefficients.(4) Compositional zonings can be interpreted by the change in the partition coefficient due to the temperature decrease.(5) Pairs from the Kamaishi mine, the largest contact metasomatic-type iron deposit in Japan, plot in the Fe-rich part on the diagram. Pairs from the Fujigatani mine, where iron oxides and sulfides are scarce, plot in the Al-rich part.