The first data are presented on the compositions of microinclusions in fibrous diamonds from the Ebelyakh placers, northeastern Siberian Platform. Their fluid/melt microinclusions are of silicate or carbonate compositions. In general, the trace element patterns for the microinclusions correspond to kimberlites and carbonatites. The major-element composition differs significantly; for example, the microinclusions are considerably enriched in K and Na. In two of the studied diamonds, the microinclusion compositions differ considerably in the cores and rims. In one of them, the composition of the medium changes from chloride–carbonate to predominantly carbonate (sample HI-90); in the other one, from carbonate to silicate (sample HI-98). Similar carbon isotope characteristics of diamonds with microinclusions of two contrasting media might suggest their crystallization from a mantle reservoir with the same carbon isotope characteristics. The geochemical features of the microinclusions in the placer diamonds revealed their relationship with protokimberlitic carbonate–silicate fluids. Such fluids might result from the metasomatic interaction of volatiles and/or the low-degree partial melting of peridotite and eclogite substrates.