The influence of phosphine and aminophosphine modifiers on the performance of 5 wt.-% Pt/C and Pt/alumina catalysts was studied in the partial oxidation of L-sorbose with molecular oxygen. The molar ratio of modifier to surface Pt atoms (Pt,) was varied between 0.1 and 1.0. In general, higher modifier/Pt-s ratios resulted in higher selectivity to 2-keto-L-gulonic acid and lower reaction rate. The activity and selectivity of Pt strongly depended on the chemical structure of phosphine. Tributhylphosphine in trace amounts (modifier : Pt-s=0.1 mol/mol; modifier : sorbose = 600 ppm) proved to be an outstanding modifier, which improved the selectivity of Pt/C by 28-37% in the whole investigated conversion range. Monitoring the catalyst potential during the reaction revealed that the oxidation state of Pt decreased by promotion, which is an indication that some side reactions leading to catalyst poisoning are suppressed.