Growth and destruction of graphene islands on Rh have been studied accounting processes on the surface and in the substrate bulk simultaneously. Atomic carbon was shown to be distributed between three equilibrium phases: graphene, solid solution in the metal substrate, and chemisorbed carbon. An increase in graphene island area results in corresponding increase in carbon concentration in the solid solution and chemisorbed phase. Atomic carbon concentrations are measured for all three phases for various stages of graphene growth and decomposition. The activation energy of atomic carbon detachment from the graphene island perimeter on Rh has been determined, Edet = 2.7 eV. Surface concentration of graphene islands has been estimated to be about 1010 per сm2.