Adsorption and chromatographic properties of oxidized and hydrogenated 'high pressure and high temperature' synthesised diamond (HPHT) are studied using high-performance liquid chromatography. The retention factors of organic cation (benzyltributylammonium chloride), weak base (aniline), weak acid (benzoic acid), strong acid (benzenesulfonic acid), hydrophobic toluene, and hydrophilic uracil are obtained at varied pH, organic solvent content, and ionic strength of mobile phase. Both adsorbents exhibited moderate polarity with a mixed-mode retention mechanism with a combination of electrostatic, hydrophobic and hydrophilic interactions. Unexpectedly, hydrogenated HPHT revealed significant anion-exchange properties under acidic conditions and cation-exchange properties under alkaline conditions, while only cation-exchange selectivity was noted for oxidized HPHT across the enntire pH range. The retention factors obtained for a set of model compounds including n-alkyl-, polymethyl-, nitro- and halogenated benzenes correlated well with their hydrophobicity (logP) values. The thermal stability of the adsorbent and immutability of retention mechanisms involved was confirmed by linear van't Hoff plots for the investigated compounds.