Black-body temperature of the cosmic microwave background radiation as a function of redshift. The star represents the measurement at z = 0 (Mather et al. 1999). Our measurements based on the rotational excitation of CO molecules are represented by red filled circles at 1.7 < z < 2.7. Other measurements at z > 0 are based (i) on the S-Z effect (blue triangles at z < 0.6, Luzzi et al. 2009) and (ii) on the analysis of the fine structure of atomic carbon (green open squares: z = 1.8, Cui et al. 2005; z = 2.0, Ge et al. 1997; z = 2.3, Srianand et al. 2000; z = 3.0, Molaro et al. 2002). Upper limits come from the analysis of atomic carbon (from the literature and our UVES sample, see Srianand et al. 2008) and from the analysis of molecular absorption lines in the lensing galaxy of PKS 1830-211 (open circle at z = 0.9, Wiklind & Combes 1996). The dotted line represents the adiabatic evolution of T_CMB as expected in standard hot Big-Bang models. The solid line with shadowed errors is the fit using all the data and the alternative scaling of T_CMB(z) (Lima et al. 2000) yielding β = −0.007 ± 0.027. The red dashed curve (resp. green dashed-dotted) represents the fit and errors using S-Z + CO measurements (resp. S-Z + atomic carbon).