When the pH of mobile phase is changed in the analysis of reverse phase mode on HPLC, the dissociation / non-dissociation state changes depending on the compound, so the retention time changes. Normally, the retention time is longer in non-dissociated state and shorter in dissociated state. This time, using Inertsil WP300 C18, we analyzed the basic compound saccharin and the acidic compound phthalic anhydride standard, and found that the elution order changed under acidic and neutral conditions. We also report that it was estimated that phthalic anhydride in the sample was partially changed to phthalic acid.