The retention behaviour of aprotic neutral molecules in Ion Interaction Chromatography was investigated. The study demonstrates that their retention, for an aqueous mobile phase (phosphate buffer pH 7.2-methanol, 85:15 v/v) containing the ion-interaction reagent (IIR) in the range 0–25 mM, decreases with increasing IIR concentration. Silanophilic interactions were accounted for by comparison of results obtained with either a silica based or a styrene-divinylbenzene based C18stationary phase, under otherwise identical conditions. The retention behaviour of neutral molecules was elucidated on the basis of the exhaustive Ion Interaction retention model and evidence was obtained to support the proposition that the decreased retention is the result of an adsorption competition between tested analytes and ion-interaction reagent for inner layer sites on the stationary phase.