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Water soluble inorganic ions such as ammonium, nitrate, and sulfate are major components of fine aerosols in the atmosphere and are widely used in the estimation of aerosol acidity. However, different experimental practices and instrumentation may lead to uncertainties in ion concentrations. Here, an inter-comparison experiment was conducted in 10 different laboratories (labs) to investigate the consistency of inorganic ion concentrations and resultant aerosol acidity estimates using the same set of aerosol filter samples. The results mostly exhibited good agreement for major ions Cl-, SO42-, NO3-, NH4+ and K+. However, F-, Mg2+ and Ca2+ were observed with more variations across the different labs. The Aerosol Chemical Speciation Monitor (ACSM) data of non-refractory SO42-, NO3-, NH4+ generally correlated very well with the filter analysis based data in our study, but the absolute concentrations differ by up to 42 %. Cl- from the two methods are correlated but the concentration differ by more than 3 times. The analyses of certified reference materials (CRMs) generally showed good recovery of all ions in all the labs, the majority of which ranged between 90 % and 110 %. Better agreements were found for Cl-, SO42-, NO3-, NH4+ and K+ across the labs after their concentrations were corrected with CRM recoveries; the coefficient of variation (CV) of Cl-, SO42-, NO3-, NH4+ and K+ decreased 1.7 %, 3.4 %, 3.4 %, 1.2 % and 2.6 %, respectively, after CRM correction. We found that the ratio of anion to cation equivalent concentrations (AE/CE) is not a good indicator for aerosol acidity estimates, as the results in different labs did not agree well with each other. Ion balance (anions - cations) calculated from SO42-, NO3- and NH4+ gave more consistent results, because of their relatively large concentrations and good agreement among different labs. In situ aerosol pH calculated from the ISORROPIA-II thermodynamic equilibrium model with measured ion and ammonia concentrations showed a similar trend and good agreement across the 10 labs. Our results indicate that although there are important uncertainties in aerosol ion concentration measurements, the estimated aerosol pH from the ISORROPIA-II model is more consistent. [ABSTRACT FROM AUTHOR]