The cleaning of an ultrafiltration polyethersulfone (PES) membrane fouled by skimmed milk was studied by ATR-FTIR showing the functional groups of fouling species and EDX coupled with SEM measuring the atomic composition. Two kinds of fouling conditions were studied: further so-called "static conditions" as performed in a beaker and "dynamic conditions" as performed on a OF loop with applied pressure. For static conditions, all milk components (proteins, lactose, ions) adsorbed on the PES surface. Lactose and salts were eliminated by water rinsing, whereas proteins were only partially desorbed by chemical cleaning at basic pH. For dynamic conditions, the cleanliness of the membrane was evaluated through two criteria: hydraulic (recovery of initial flux) and chemical (no more contaminants detected). The hydraulic cleanliness of the membrane was achieved whereas the membrane initial surface state was not restored.