Measurements of the gas phase proton transfer equilibrium [1]: AH + B− = A− + BH with a pulsed electron high pressure mass spectrometer (PHPMS) lead toanddata. These relative acidities are converted to the absolute acidities [2]: AH = A− + H+bycalibration of the relativeandscale to the known values forandof a reference compound(HCl). Earlier determinations that included 16 aliphatic AH are extended in the present work and provide data for 47 aliphatic carboxylic acids. These include halogen, OH, CH3O, C2H5O, PhO, and NH2substituted acetic acids, and halogen substituted, unsaturated, and cyclic RCO2H of higher carbon number. The effects of the various substituents on the gas phase acidity are briefly examined.Keywords: acidities in gas phase, proton transfer equilibria, ion-molecule equilibria, mass spectrometry.