We created a model system made up of PU networks containing reactive surfactant additives in small amounts (<1%), brought in during the synthesis and able to reorient when changing the surrounding medium. The surfactants were made of fluorinated chains, sulfonamide kneecaps, and alkyl chains with a hydroxyl function able to react with the network isocyanates. The basic PU were made of a macrodiol, a diisocyanate and a triol (cross-linking agent). We were able to change their bulk glass transition easily and therefore their surface mobility by varying the respective ratios of diol and triol (a rubbery one, T-g = -20 degreesC, and a glassy one, T-g = 40 degreesC). The reactive surfactants were used to give a hydrophobic nature to the surface of PU networks when in contact with air. Several characterizations were performed, including wetting and dewetting measurements, to prove that less than 1 wt % of the fluorinated reactive additives is sufficient to confer a very hydrophobic nature to the network surface without changing its bulk properties. The influence of the network glass transition on the top surface reorganization was also investigated.