The rhodium-catalyzed hydroformylation with the diphosphites P[O(2,2‘-(4-X-6-Y-C6H2)2O][O(2,2‘-(4-X-6-Y-C6H2)2)OP][O(4-Z-C6H4]2 (X = Y = tert-butyl, Z = H (1), Z = OMe (2), Z = C6H5 (3), Z = Cl (4); X = OMe, Y = tert-butyl, Z = H (5)), [PO(2,2‘-(4,6-(tert-C4H9)2C6H2)2)O]2R [R = O(CH2)2O (6), R = O(CH2)3O (7)], [PO(2,2‘-(C6H4)2)O]2R, R = O(2,2‘-(4-MeO-6-tert-C4H9C6H2)2O (8), R = O(2,2‘(4,6-tert-C4H9)2C6H2)2O (9)], and [PO(2,2‘-(4,6-(tert-C4H9)2C6H2)2)O]2[O(2,2‘-(C6H4)2)O] (10) as ligands is studied with oct-1-ene and styrene as substrates. For oct-1-ene the highest normal to branched ratio obtained is 48 (5). For styrene the product selectivity depends strongly on the reaction temperature; a branched to normal ratio of 19 is found for 7 when T = 40 °C vs a branched to normal ratio of 0.19 for 1 when T = 120 °C. A bulky and bisequatorially (ee) coordinating diphosphite is required to obtain a high regioselectivity for linear aldehydes, while flexible diphosphites or equatorially−axially (ea) coordinating diphosphites lead to an enhancement of the formation of branched aldehydes. The hydroformylation of oct-1-ene has a first-order dependency in the oct-1-ene concentration, the order in CO is approximately −0.65, and the order in H2 is approximately 0.2. This is consistent with a kinetic scheme in which alkene addition is the rate-determining step. The crystal structures of RhAcac(4) and RhH(CO)2(4) (11), are presented. The hydrido ligand in 11 could not be located. The structure reveals a distorted TBP with 4 ee coordinated, the P(1)−Rh−P(2) angle being 115.95(9)°. The distortion is indicative of a crowded rhodium center which explains the obtained high linearity of the hydroformylation products.