Two graphite deposits in the southwestern Grenville Province are investigated to evaluate the origin of graphitic carbon and to test if the graphite mineralization is syngenetic or epigenetic. Graphite mineralization in the Bissett Creek deposit is characterized by homogeneously distributed and disseminated graphite flakes (approximately 1 to 5 mm in size and 2 to 10 vol.%) within graphitic gneisses. The graphite flakes are intergrown with metamorphic minerals, most notably biotite. The Montpellier graphite showing in Québec contains graphite concentrations of up to 20 vol.%. In contrast to the disseminated and homogenously distributed graphite in the Bissett Creek deposit, graphite mineralization at Montpellier forms lenses of variable sizes that occur at the top of a calc-silicate unit and as graphite-rich lenses in biotite-sillimanite-rich paragneiss. The δ13C of graphite ranges from –29 to –17 ‰ at Bissett Creek and from –18 to –14 ‰ at Montpellier. Carbon isotope compositions of graphite from both deposits support a biogenic source for the carbon and the spread in δ13C can be generated through Rayleigh fractionation. A minor contribution of inorganic carbon from the devolatilization of carbonate minerals is possible at Montpellier. Mineralization at Bissett Creek and Montpellier is interpreted to represent syngenetic graphite mineralization from organic-rich material during high-temperature metamorphism.