Constant volume dilution of CeRu 2Si 2 has been attempted by pseudo-ternary compounds Ce 1- x(La 0.63Y 0.37) xRu 2Si 2. The metamagnetic transition of CeRu 2Si 2 is found to disappear at weak dilution ( x ≅ 0.2), with little shift in the critical field. In understanding the ground state of URu 2Si 2, single site properties of U 4+ have been studied in Th 1- xU x Ru 2Si 2, x ⩽ 0.1. Magnetic measurements strongly suggest that the crystalline field ground state of U 4+ is a magnetic doublet, exhibiting an unconventional Kondo effect with logarithmically divergent magnetic susceptibility at low temperature. Both results indicate that the inter-site effects are crucial in understanding the magnetic behavior of these heavy electron systems.