The proton affinities of furan, 2-, 3-, and 4-methylphenol, and the related anisoles have been determined with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The proton affinity of furan is redetermined to be 812 kJ mol(-1) on the basis of the experimental equilibrium constant for the proton transfer to acetone. The present value is significantly higher than that recommended in the literature (803 kJ mol(-1)), but in agreement with ab initio G3(MP2) calculations, which suggest a proton affinity of 814 kJ mol(-1) for the 2-position in furan. The determination of the equilibrium constant for the reaction between a protonated methylphenol or methylanisole and a suitable reference base results in the following proton affinities: 2-methylphenol, 832 U mol(-1); 3-methylphenol, 841 kJ mol(-1); 4-methylphenol, 814 kJ mol(-1); 2-methylanisole, 850 kJ mol(-1); 3-methylanisole, 860 kJ mol(-1); and 4-methylanisole, 841 kJ mol(-1). Calculations at the G3(MP2) level indicate that the 4-position is the most basic site in the 2- and 3-methyl-substituted phenols, whereas almost the same proton affinity is obtained for the 2- and 4-position in 4-methylphenol. The G3(MP2) proton affinity for the most basic site in a given methylphenol is in agreement with the present experimental values.