AbstractA cationic rhodium(I) complex, viz. Rh NBD diphos+ 4‐CH3‐C6H5SO3− [NBD = norbornadiene, diphos = (C6H5)2 P‐CH2‐CH2‐P(C6H5)2], has been used as a homogeneous catalyst for the hydrogenation of soybean oil in acetone solution. This complex acts almost in the same way as the corresponding ones with ClO4− or PF6− as conuterions, i.e., it gives high polyene selectivity and low formation oftrans isomers. Because of the somewhat stronger basic character of the p‐toluene‐sulfonate ion compared with the perchlorate and hexafluorphosphate ions, the relative proportion of reaction via the so‐called monohydride path is larger in the present case. When the ionic complex, Rh NBD diphos+, is bound to a solid support, e.g., to the anionic sites of sulfonated polystyrene resins, a nearly total lack of catalytic activity is observed. Possible reasons for these effects are discussed‐in terms of π‐arene‐metal binding and covalent coordination of the sulfonate group.