Gas-phase C/O ratio as a function of distance from a solar-mass star for a disc characterised by α = 10⁻³. Red lines represent our fiducial model where refractory organics thermally decompose into C₂H₂ (T_sub = 70 K), while green lines show the results of a model where they decompose into CH₄, a more volatile species (T_sub = 30 K). The horizontal dashed grey line represents the initial C/O ratio. The sublimation lines of the main species contributing to the C/O ratio are indicated with vertical lines.