Since in ion-interaction chromatography the dependence of the retention on the concentration of the ion-interaction reagent (IIR) follows different behaviours for the different analytes, the adjustment of IIR concentration can usefully be used to optimize resolution. As an example, resolution between nitrate and p-aminophenol was investigated as a function of IIR concentration and mobile phase flow-rate. The IIRs used are an aqueous solution of octylamine, brought to pH=6.4 with o-phosphoric acid.