The CO(1-0) and [C i](1-0) emission lines are well-established tracers of cold molecular gas mass in local galaxies. At high redshift, where the interstellar medium is likely to be denser, there have been limited direct comparisons of both ground-state transitions. We present a comparison of [C i](1-0) and CO(1-0) emission in 20 unlensed dusty, star-forming galaxies at z ≥ 2-5. The CO(1-0)/[C i](1-0) ratio remains constant up to z = 5, supporting the reliability of [C i](1-0) as a gas-mass tracer. We use the CO(1-0), [C i](1-0), and 3 mm dust continuum measurements to cross-calibrate their respective gas mass conversion factors, finding no dependence of these factors on either redshift or infrared luminosity. Radiative transfer modeling shows that the warmer cosmic microwave background (CMB) at high redshift can significantly affect the [C i] as well as CO emission, which can change the derived molecular gas masses by up to 70% for the coldest kinetic gas temperatures expected. Nevertheless, the magnitude of the CMB effect on the CO/[C i] ratio is within the known scatter of the L CO ′ − L [ CI ] ′ relation. Precisely determining the CMB effect on individual line intensities would require well-sampled spectral line energy distributions to robustly model the gas excitation conditions. Finally, we note that adopting a variable CO gas-mass conversion factor for different galaxy populations implies [C i](1-0) and dust conversion factors that differ from canonically assumed values. However, the revised conversion factors are consistent with expectations for (super)solar metallicities likely to be found in high-redshift dusty galaxies.

The molecular gas in the interstellar medium (ISM) of star-forming galaxy populations exhibits diverse physical properties. We investigate the CO excitation of 12 dusty luminous star-forming galaxies at 2-4 by combining observations of the CO from to. The spectral line energy distribution (SLED) has a similar shape to NGC 253, M82, and local ultra-luminous infrared galaxies, with much stronger excitation than the Milky Way inner disc. By combining with resolved dust continuum sizes from high-resolution 870 m ALMA observations and dust mass measurements determined from multiwavelength spectral energy distribution fitting, we measure the relationship between the CO SLED and probable physical drivers of excitation: star-formation efficiency, the average intensity of the radiation field, and the star-formation rate surface density. The primary driver of high-CO excitation in star-forming galaxies is star-formation rate surface density. We use the ratio of the CO(3-2) and CO(6-5) line fluxes to infer the CO excitation in each source and find that the average ratios for our sample are elevated compared to observations of low-redshift, less actively star-forming galaxies and agree well with predictions from numerical models that relate the ISM excitation to the star-formation rate surface density. The significant scatter in the line ratios of a factor within our sample likely reflects intrinsic variations in the ISM properties that may be caused by other effects on the excitation of the molecular gas, such as cosmic ray ionization rates and mechanical heating through turbulence dissipation.