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.