Reduced carbon in the mantle is commonly thought to be chiefly in diamond, but experiments suggest that at >250 km the mantle contains small amounts (0.1-1 %) of FeNi alloy. [1, 2, 3]. Thus, alloy may be a significant host of reduced C , but little is known about C solubility of FeNi alloy under mantle conditions. To determine the carbon solubility in FeNi alloy and melt, we conducted experiments in the system Fe-Ni-C with bulk compositions having 5 wt. % C and variable Fe/(Fe+Ni) at 3 to 7 GPa and 1000 - 1400°C. Experiments at 3 GPa and 1000-1250 °C were performed in an end loaded piston cylinder apparatus; those at 5 and 7 GPa and 1200-1400°C were performed in a 1000 ton Walker-style octahedral multianvil. At 3 GPa, Fe-rich melts contain up to 4.5 wt. % C, but Ni-rich (>25 mole% Ni) compositions remain subsolidus at 1250°C. The solubility of carbon in pure Fe and Ni metal are 2 wt. % and 1 wt. % respectively, but in the alloy passes through a minimum of 0.4 wt. % for Fe0.2Ni0.8. Assuming that these concentrations apply at higher temperatures and pressures (as will be tested by future experiments) allows a first estimate of the potential storage of C in FeNi alloy in the mantle. If the mantle at 250 km contains 0.1-0.2 wt.% Ni-rich (Fe0.4Ni0.6) alloy, increasing with depth to 1 wt.% Fe-rich (Fe0.88Ni0.12) alloy at 700 km [1,2], then maximum storage of C in alloy rises from 5 ppm in the deep upper mantle to 180 ppm in the shallow lower mantle. For mantle similar to the MORB source, with ~10-30 ppm C , alloy cannot store all C in the deep upper mantle but can in the lower mantle. For OIB sources with 33-500 ppm C , complete storage in alloy is less likely. Additional phases will be diamond in the upper mantle, as our experiments and previous work  indicate that carbide is not stable in equilibrium with Ni-rich alloy, and carbide melt in the lower mantle.  Frost et al (2004) Nature 428 409-412  Frost and McCammon (2008) EPSL 36 389-420  Rohrbach et al. (2011) J.Petrol 52 #717-731  Dasgupta et al. (2009) GCA 73 6678-6691  Dasgupta and Hirschmann (2010) EPSL 298 1-13  Romig and Goldstein (1978) Metal Trans. Met. AIME 9a 1599-1609.