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tokens: gas and solids in discs (e.g. Öberg et al. 2011 ; Madhusudhan et al. 2014 ). We first explore the evolution of the C/O ratio in models without radial drift (Fig. 6 ). In these models, the total abundance (gas and ice phase) of carbon and oxygen bearing species does not change, only the partitioning of these species between the gas and ice phases. Figure 6. Evolution of the C/O ratio for models with viscous transport, but without radial drift or diffusion. For comparison the left-hand panel shows the evolution without transport. The solid lines denote the gas-phase abundances, while the dotted lines show the ice abundances. The changes in composition driven by chemical processes are best seen in the model without transport (and α = 10 −3 ). Inside the water ice line (0.5 au), all of the major carbon and oxygen are in the gas phase and thus chemical processing does not change the gas-phase carbon or oxygen abundance. Between the water and CO 2 ice lines (1.5 au) the gas-phase carbon and oxygen abundance is dominated by CO and CO 2 , with a small contribution from hydrocarbons (CH 4 and C 2 H 2 – a proxy for C 2 H 6 ; see Section 3.1 ). Here, the C/O ratio increases as CO 2 is destroyed, producing CO and O, with O eventually forming H 2 O, which freezes out. Between the CO 2 and CH 4 ice lines (10 au), CO and CH 4 are initially the main gas-phase carbon and oxygen carriers. CH 4 gets converted to larger hydrocarbons (C 2 H 2 ), which freeze out causing the C/O ratio to reduce to 1. The spike in C/O ratio near the CO 2 ice line is due to the slightly lower binding energy of C 2 H 2 than CO 2 (note that C 2 H 6 also has a lower binding energy than CO 2 , so Eistrup et al. ( 2018 ) see a similar spike in C/O). Beyond the CH 4 ice line CO is always the dominant the gas phase carbon and oxygen carrier, so the C/O ratio does not evolve. The nitrogen abundance evolution can be derived from Fig. 5 and is slow, thus the C/N of N/O ratio evolution will be driven primarily by the evolution of carbon and oxygen, respectively. As the speed of the viscous transport increases, the changes in composition driven by chemical reactions become restricted to regions of the disc at larger and larger radii. Already at α = 10 −3 , the effect of transport is first noticeable between the H 2 O and CO 2 ice lines, since CO 2 no longer depletes (see also Bosman et al. 2018a ), while at α = 10 −2 the carbon and oxygen abundance hardly evolve. At α = 10 −2 , the inward movement of the ice lines as the disc cools generates the only significant changes in abundance ratio. The influence of radial drift on the bulk abundances is sensitive to whether radial drift or viscous evolution is faster. When radial drift is included we see spikes in the gas-phase C/H ratio at α = 10 −3 (Fig. 7 ), as reported by Booth et al. ( 2017 ). These spikes are driven by volatile species being left at their snow lines when grains drift past them. We also see a peak in the C/O ratio inside of 10 au,