Diamond microinclusions provide the unique opportunity to study the composition of the mineral-forming medium. The paper presents the first data on the composition of cloudy microinclusions in the cores of octahedral diamonds from the Internatsional’naya pipe. These zones are of cuboid shape and have a fibrous internal structure. Here, the microinclusions form a continuous trend from chloride–carbonate to carbonate composition. Their composition slightly overlaps that of the microinclusions in cuboids from the same pipe, but in a carbonate-enriched zone. Also, data on N aggregation suggest that the early cubic zones which formed before octahedra crystallized at higher temperature or spent considerably more time in the mantle than cubic crystals and fibrous rims of variety IV diamonds.