CO luminosity to gas mass conversion factor, α_CO, versus FIR luminosity, for the seven Planck’s dusty GEMS with direct α_CO measurements using the molecular gas masses inferred from the [C I](1-0) line fluxes of Nesvadba et al. (2018, red stars). All but one GEMS are consistent with the usual ULIRG value α_CO = 0.8 M_⊙ (K km s⁻¹ pc²)⁻¹ (lower dashed line). Other points show α_CO estimates in the literature, for the strongly lensed SMGs from the SPT survey (blue points), high-redshift unlensed dust-obscured starbursts (green points), QSOs (yellow points) and main sequence galaxies (black squares, Aravena et al. 2016, and references therein). The relative uncertainties on α_CO obtained for these high-redshift samples are comparable to those of the GEMS.