A technique for the routine determination of the nitrogen contents and the δ¹⁵N and δ¹³C values of diamonds has been developed. δ¹³C and δ¹⁵N measurements have been performed on coated, cubic and octahedral diamonds from Australia, Africa, Siberia and N. America. Amongst the octahedral diamonds two broad groups can be distinguished. Group O₁ diamonds have δ¹³C values between -3 and -7v (similar to most other mantle samples) and contain nitrogen generally depleted in ¹⁵ relative to AIR; δ¹⁵N ca.-5‰ (range +5 to -13‰). Group O₂ diamonds have variable δ¹³C values (-21 to -2‰) and contain N generally enriched in ¹⁵ relative to AIR; ca. +5‰ (range -5 to +16‰). The cubic coatings on diamonds from Zaire, Sierra Leone, Siberia, Angola and Botswana and cubic diamonds from Zaire have a restricted range in isotopic composition (δ¹³C -5 to -1.5‰, δ¹⁵N -2 to -8‰). Both group O₁ and O₂ diamonds were present as cores within these samples. The coats of coated diamonds have been interpreted as being phenocrystic overgrowths. The isotopic uniformity of diamond coats, on a regional scale, suggests the existence of a homogeneous C and N reservoir underlying the continental lithosphere. This reservoir supplies the volatiles associated with kimberlite eruptions. Coated diamonds occur in Phanerozoic kimberlites which implies that, by this time, the extreme carbon isotope heterogeneity within the mantle (+3 to -35‰) was restricted to the continental lithosphere. The isotopic characteristics of the coats are similar to the O₁ diamonds. Since diamonds from Finsch and Premier (both O₁) have been dated at 3.3 Ga and 1.2 Ga respectively there appears to have been little change in the δ¹³C and δ¹⁵N value of the uniform mantle reservoir since the mid-Archean. The origin of the isotopic characteristics of the group O₂ diamonds is uncertain, however it is unlikely that they are reflecting relict primordial heterogeneity.