Oxidized starch is heavily deployed as a surface sizing agent in the paper industry. The most common oxidation methods use transition metals as catalysts and perchlorines or iodates as oxidants, and thus a large amount of inorganic waste is produced. In the present work, iron tetrasulfophthalocyanine (FePcS), a cheap iron complex is used as a catalyst, and hydrogen peroxide (H2O2) as an oxidant, resulting in a waste free method for starch oxidation. The oxidation is performed in a semibatch mode, constantly feeding H2O2 into the reactor and the pH is kept constant by adding NaOH. Gas evolution was monitored by measuring gas flow in the outlet and performing mass spectrometric analysis. The samples were taken during experiments, and the carboxyl (COOH), carbonyl (CO) contents as well as hydrogen peroxide concentration, were analyzed resulting in kinetic data. At neutral pH (7), the catalyst is not well activated leading to accumulation of H2O2, whereas under alkaline reaction conditions (pH 10), the H2O2 was rapidly decomposed resulting in a COOH value of 1.6/100 AGU and CO value of 3.0/100 AGU, but with an extensive loss of the solid starch (yield = 67 wt.%) due to depolymerization and decomposition. Respectively, no carbon dioxide but only oxygen was detected in the formed outlet gas when using FePcS under alkaline solutions, whereas with FeSO4 in acidic solutions only CO2 was detected without any oxygen.