The carbon isotopic composition of 66 inclusion-containing diamonds from the Premier kimberlite, South Africa, 93 inclusion-containing diamonds and four diamonds of two diamond-bearing peridotite xenoliths from the Finsch kimberlite, South Africa was measured. The data suggest a relationship between the carbon isotopic composition of the diamonds and the chemical composition of the associated silicates. For both kimberlites similar trends are noted for diamonds containing peridotite-suite inclusions (P-type) and for diamonds containing eclogite-suite inclusions (E-type): Higher δ13C P-type diamonds tend to have inclusions lower in SiO2 (ol), Al2O3 (opx, gt), Cr2O3, MgO, Mg (Mg + Fe) (ol, opx, gt) and higher in FeO (ol, opx, gt) and CaO (gt). Higher δ13C E-type diamonds tend to have inclusions lower in SiO2, Al2O3 (gt, cpx), MgO, Mg (Mg + Fe) (gt), Na2O, K2O, TiO2 (cpx) and higher in CaO, Ca (Ca + Mg) (gt, cpx). Consideration of a number of different models that have been proposed for the genesis of kimberlites, their xenoliths and diamonds shows that they are all consistent with the conclusion that in the mantle, regions exist that are characterized by different mean carbon isotopic compositions.