A new method using fluorescence labeled proteins was developed to determine the quantitative amount of specific protein at the powder surface. The method is based on steady-state fluorescence measurements of pyrene labeled proteins with oxygen gas phase quenching at the powder surface. The surface load of protein was measured for spray-dried dextran powders containing bovine serum albumin (BSA). The results show a patchwise surface load of about 1.3 mg/m2 at a concentration of 0.33 % (dry weight) BSA in the powder. The patchwise surface load stays constant with increased BSA concentration in the powder.