Mn-Ce-O composite catalysts are being widely advocated in sub-and supercritical catalytic wet oxidation (WETOX) for the mineralization (i.e. complete conversion to CO2) of toxic organics contained in aqueous streams. Catalytic WETOX of CHO-containing compounds over Mn-Ce-O catalysts is accompanied with undesirable heavy polymers building upon the catalyst surface, thereby leading to severe loss in catalytic activity and mineralization selectivity. Hence, new potassium-doped Mn-Ce-O mixed oxide catalysts (Mn/Ce atomic ratio = 1) synthesized by co-precipitation/impregnation were tested for the destruction of phenolic model wastewaters in a batch slurry reactor using oxygen and very mild reaction conditions. Remarkable improvements in mineralization selectivity were attained upon addition of potassium. Complete removal of the organic pollution, with mineralization selectivity exceeding 95%, was achieved within 10-20 min over fresh K-Mn-Ce-O catalyst. Virtually, total elimination of organic carbon was also achieved using the same catalyst 3 times without any regeneration. Likewise, the stability to leaching of the WETOX catalytic ingredients was very high as confirmed by analysis of the treated solutions. Characterization of fresh and used Mn-Ce-O and K-Mn-Ce-O catalysts was conducted using nitrogen adsorption, temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO), infrared spectroscopy (FTIR), CHN elemental analysis and X-ray diffraction (XRD) techniques. These techniques corroborated the drastic reduction in carbonaceous deposits on the catalyst surface and the significant enhancement in stability of the K-doped catalysts. These improvements were discussed in terms of modification of the redox properties of cerium and manganese through interactions with potassium in the doped Mn-Ce-O catalysts.