Carbon-bearing fluids and condensed carbon are common on and inside planetary bodies. Understanding the mechanisms capable of transferring carbon from fluids into solids and vice-versa is central in many fundamental and applied research targets within the Earth and Planetary Sciences. A broad range of applications can benefit from the thermodynamic properties of carbon-oxygen-hydrogen (COH) systems. As an example, the precipitation of natural graphite or diamond can be modeled within the COH thermodynamic systems. Because the evolution of COH fluids implies isotopic fractionation among different species, carbon stable isotopes can be used in conjunction with thermodynamic calculations to reconstruct graphite or diamond formation mechanisms, or the evolution of fluid species such as carbon dioxide or methane. Thermotopes-COH is a Python-based graphical user interface (GUI) software for computation of thermodynamic and carbon isotopic modeling in the C-O-H system within the 0.1-5 GPa and 300-900°C range. The software allows generic and process-oriented thermodynamic and carbon stable isotope calculations including fluid mixing, fluid-graphite/diamond interactions with changing pressure or temperature, and fluid desiccation. This versatile numerical tool is designed to model processes of dissolution/precipitation of graphite or diamond. Along with a description of the software functions, this contribution also provides practical examples.