Poly(methacryloyloxy ethyltrimethylammonium chloride) (PMETAC), poly(sulfopropylmethacrylate potassium salt), or poly(N-isopropyl acrylamide) (PNIPAM) brushes are synthesized by means of the atom transfer radical polymerization technique from gold surfaces coated with a monolayer of the initiator ω-mercaptoundecyl bromo isobutyrate. The brush growth is followed in situ and in real time by the combination of quartz crystal microbalance with dissipation technique (QCM-D) and spectroscopic ellipsometry in a single device. The combination of QCM-D and ellipsometry allows for the simultaneous determination of both the acoustic mass, macous, comprising the mass of the polymer and the solvent, and the optical mass, mopt, which corresponds to the polymer mass alone. Brush hydration is calculated from the difference between the values obtained for macous and mopt for each polymer synthesized. Brush hydration is then used to quantify the percentage of water released in the brush during collapse; a 30–40% release of water for PMETAC and PSPM brushes in 1 M NaCl and 80% for PNIPAM brushes when the temperature is increased to values above the lower critical solution temperature is observed.