Relative proton affinities are usually measured by means of high pressure mass spectrometry. In this work ion mobility spectrometry was used to determine proton affinities. The standard molar enthalpy change DeltaH(M)(0) for the reaction MH+N reversible arrow M + NH+ was found to be: - (56.3 +/- 0.8) kJ . mol(-1), when M was ethyl acetate and N ethanol; -(27.8 +/- 0.5)kJ . mol(-1), when M was acetophenone and N ethyl acetate; -(10.6 +/- 0.2)kJ . mol(-1), when M was cycloheptanone and N ethyl acetate; -(66.1 +/- 1.2)kJ . mol(-1), when M was dimethyl methyl phosphonate and N ethyl acetate; and, -(56.7 +/- 10. 9)kJ . mol(-1), when M was dimethyl methyl phosphonate and N cycloheptanone. These values are internally consistent and in good agreement with results obtained with high pressure mass spectrometry. On the basis of proton affinities of cycloheptanone and ethyl acetate, an absolute proton affinity of (902 +/- 1.2)kJ . mol(-1) has been determined for dimethyl methyl phosphonate. (C) 2003 Elsevier Science Ltd. All rights reserved.