The dependences on temperature of the rate coefficients for the endothermic proton transfer reactions of HBr+ with CO2 and CH4 have been obtained in a variable-temperature selected ion flow tube. The measurements have been used to determine the 300 K proton affinity of CO2, P.A.(CO2), =128.5±1.0 kcal mol-1, utilizing the literature value of P.A.(Br)=131.8 kcal mol-1, obtained from the dissociation energy of HBr+, as a primary standard. The proton affinity difference between CO2 and CH4 has been substantiated by equilibrium constant measurements as a function of temperature for proton transfer between CO2 and CH4. Similar equilibrium constant measurements have been used to determine the proton affinities of HCl, N2 O, HBr, and CO, giving a proton affinity ladder ordered (in kcal mol-1) as CO(141.4), HBr(138.8), N2 O(137.3), HCl(133.0), Br(131.8), CH4 (130.0), and CO2(128.5). Proton affinities have also been determined for Br2(140.0), NO(127.0), and CF4(126.5), the last two values being obtained from selected ion flow drift tube measurements. An upper limit to P.A.(SF6) of 127 kcal mol-1 has been inferred from the instability of SF6 H+ towards dissociation into SF+5 and HF.