The composition of an argon plasma in presence of carbon and
hydrogen is calculated under assumption of thermal equilibrium in temperature range 500−6000 K. The calculations are performed for a plasma treated as a single phase system but also for the plasma in equilibrium with carbon in the form of solid graphite. For different C/H ratios the highest possible temperatures at
which carbon still appears in solid state form are determined. The results of plasma composition obtained for the plasma in presence of solid graphite are compared to those obtained neglecting the existence of the solid phase.