Catalytic activity of platinum catalysts such as Pt/graphite, Pt/TiO2, Pt/Al2O3, and Pt/active carbon was studied using a slurry phase CSTR. Three model reactions, namely, phenol, maleic acid, and malonic acid oxidation were investigated in the temperature range from 120 to 170 C and at a total reactor pressure of 1.7 MPa. Platinum on graphite was found to be most suitable for aqueous phase oxidation of phenol, maleic acid, and malonic acid. Complete conversion for both phenol oxidation as well as maleic acid oxidation to CO2 was observed with Pt/graphite at stoichiometric oxygen excess close to 0% and at 150 C. Deactivation due to over-oxidation is gradual for Pt/graphite with a metal dispersion of 5.3% as compared to Pt/TiO2, Pt/Al2O3 and Pt/AC, which have metal dispersions of 15.3%, 19.5% and 19.0%, respectively. It was further found that in the presence of Pt/graphite catalyst and oxygen, malonic acid reaction proceeds via non-catalysed decarboxylation, and catalytic decarboxylation to CO2 and acetic acid, and catalytic oxidation to CO2 and H2O. Acetic acid was found to be difficult to oxidise at temperatures below 200 C.