The effects of transition metal ions as components of the mobile phase on the retentions in HPLC of 2-aminophenol and selected α-diketones were studied on both ODS-silica and porous polymer columns. The compounds were used as model compounds in an investigation which set out to derive a detailed understanding of the mechanisms Involved when metal ions are used to selectively modify the retention of compounds capable of interaction. This technique, which has parallels with ion-pair chromatography, provides another method by which the conditions of a separation may be altered and the selectivity adjusted to give better resolution.