A condensed monolayer of carbon on the (111) surface of C-doped nickel single crystals has been observed at different bulk doping levels in the range of 10-1 to 1 at%. This phase persists over a range of temperatures of more than 100 K; it is bounded at high temperatures by the formation of a dilute carbon phase at TS and at a lower temperature, TP, by the formation of a thick graphite precipitate. The ratio TS TP is about 1.11 at all doping levels investigated. The dependence of the precipitation and the phase transition temperatures on the bulk doping level allow the binding energy per carbon atom in the monolayer phase relative to that in the bulk solution to be determined; it is approximately 10% greater than for bulk graphite with a value of 0.55 eV. The partial atomic entropy in the monolayer phase was also determined; it is about 0.03 k greater than for bulk graphite.