Computations of thermodynamic equilibrium conditions for depositing carbon in various crystallographic forms are carried out for C-H-O reformates from methane and LPG. Selected examples of reactions accompanying oxidation or thermal decomposition of C1 to C-4 hydrocarbons are presented, followed by theoretical fundamentals for chemical equilibrium. Table values of the standard chemical potential of the reactants are cited along with respective reaction equilibrium constants. Thermodynamic critical conditions for deposition of several carbon allotropes from gaseous reformates originated from partial oxidation with either air or pure oxygen are also quantified. Computation results are presented graphically in terms of the oxygen-to-carbon ratio, O/C, against temperature for varied pressure and oxidant. Zones of either carbon deposition or carbon-free are specified for graphite or one of the two carbon nanotubes, CNTs. The maximum effect of increase in the critical O/C for a CNT against pure graphite is estimated at 85%. (C) 2017 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.