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Gas phase proton transfer equilibria involving compounds in the basicity regime near CO have been examined using pulsed electron beam high pressure mass spectrometric techniques. Using the well established proton affinity of C2H4 of 162.6 kcal mol−1 as a reference standard a new proton affinity for CO of 145.6 kcal mol−1 is established. The disagreement between this value and those obtained by photoionization mass spectrometric appearance potential measurements of HCO+ ions from compounds of the type HCOR is interpreted as being due to an activation energy barrier for the reverse process: HCO++R →HCOR+. This barrier results from the significant geometry change involved in HCO+ formation. Correlations of proton affinity of O1S binding energies have been used to establish the proton affinity of the oxygen site of CO as 109 kcal mol−1. This result implies that HOC+ can be readily formed by reaction of H+3 with CO in interstellar clouds.