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We report Fe57 Mössbauer spectroscopy studies of the effect of carbon doping on the magnetic properties of the Fe sites of R2Fe17-xMxCy, and of the presence of multiple phases in these systems. In this study two systems, Sm2Fe14Al3Cy, where y=0, 1.0, 2.0, and 3.0, and Nd2Fe14.54Si2.46Cy, where y=0, 0.432, and 2.544, were measured at room temperature and analyzed. The ternary compounds R2Fe17-xMxCy studied here have the rhombohedral Th2Zn17 structure with c-axis anisotropy. The Al and Si site occupation used in the fitting were taken from previous results. Analysis of the Mössbauer spectra showed that the hyperfine fields for the different sites decreased in the order 6c, 9d, 18f, and 18h. The average hyperfine field for Sm2Fe14Al3Cy was found to increase with increasing carbon concentration, which is in qualitative agreement with magnetic measurements. For Nd2Fe14.54Si2.46Cy the average hyperfine field was found to increase for low C concentration and decrease for high C concentration; this behavior is in agreement with our magnetization measurements. The measured average isomer shift relative to α-iron was found to increase with y. In these compounds alloying with C and/or M can improve the magnetic properties such as Curie temperature and magnetization which make them potential candidates for permanent magnet development.