An increasingly popular anion exchange chromatography method <Decesari et al., 2000> was used to separate organic matter in fog samples and water soluble organic carbon (WSOC) extracted from aerosol samples according to acidity. Analysis of both fog and aerosol WSOC samples showed results similar to previous studies, with peaks typically identified as corresponding to neutral/basic compounds, mono- and dicarboxylic acids and polyacids. In one example this approach was shown to classify a total of 82% of the WSOC into these three chromatographic fractions. Challenges to the classification scheme were made by injection of single compound solutions. Compounds were chosen to be representative of compounds observed in atmospheric samples. In many cases test compounds eluted in fractions other than expected based on classifying compound structure according to the three classes outlined above. The impact of classification errors is impossible to quantify without understanding the complete organic speciation of a sample, but is serious enough that researchers relying on this method to provide a suitable model for organic aerosol composition should interpret results with caution. The most severe problems are likely for fog samples, due to a prevalence of low molecular weight carboxylic acid and carbonyl compounds which exhibit greater tendency to be misclassified.