The thermochemical acid/base properties of the six dihydroxybenzoic acids (x,y-DHB) as prototypical matrices used in matrix-assisted laser desorption/ionization (MALDI) have been investigated. The ground-state gas-phase basicities (GB) of the six DHB isomers and the gas-phase acidities (ΔG a c i d ) of the corresponding radical cations ([x,y-DHB] . + ) have been determined by Fourier-transform ion cyclotron resonance mass spectrometry employing the thermokinetic method. The gas-phase basicities vary from 814 kJ mol - 1 for the least basic isomer, 3,5-DHB, to 831 kJ mol - 1 for the most basic isomer, 2,4-DHB. The obtained gas-phase acidities of the corresponding radical cations vary from 815 kJ mol - 1 for the most acidic species, 3,4-DHB, to 858 kJ mol - 1 for the least acidic one, 2,5-DHB. The results indicate that ground-state proton transfer from the matrix radical cations to the analyte may play a role in the ionization process of MALDI, whereas proton transfer from protonated matrix molecules can be excluded.