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 oxidn. were investigated in the temp. 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 aq. phase oxidn. of phenol, maleic acid, and malonic acid. Complete conversion for both phenol oxidn. as well as maleic acid oxidn. to CO2 was obsd. with Pt/graphite at stoichiometric oxygen excess close to 0% and at 150 °C. Deactivation due to over-oxidn. 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%, resp. It was further found that in the presence of Pt/graphite catalyst and oxygen, malonic acid reaction proceeds via non-catalyzed decarboxylation, and catalytic decarboxylation to CO2 and acetic acid, and catalytic oxidn. to CO2 and H2O. Acetic acid was found to be difficult to oxidise at temps. below 200 °C.