Abstract The number of oxygen-containing functional groups (OCFGs) on the surface of a commercial activated carbon (AC) was maximized using ammonium persulfate ((NH4)2S2O8, APS) as an oxidant. Effects of the oxidation conditions, including temperature, time and concentration of APS, on physical and chemical properties of the treated AC were examined to develop a relationship between the oxidation conditions and surface chemistry of AC. A maximum amount of oxygen, 18.7 mmol/g, on the AC was achieved by oxidation with 2.0 mol/L APS solution and an APS/AC weight ratio of 9.1 at 60 °C for 3 h. Temperature-programmed desorption coupled with mass spectrometry (TPD–MS) was used to identify and quantify OCFGs on the AC samples. It was found that using the TPD–MS method currently reported in the literature to quantify various OCFGs on the carbon surface on the basis of TPD–CO2 and TPD–CO profiles may significantly underestimate the number of carboxyl groups, and overestimate anhydrides and lactones due to some in situ reactions of OCFGs during the TPD–MS analysis. The amount of H2O and CO released in the temperature range corresponding to decomposition of carboxyl groups should be taken into account when estimating the amount of carboxyl groups on AC.