In-situ spectroscopic ellipsometry was used to investigate the swelling behavior of thin supported zwitterionic polymers based on sulfobetaine methacrylate and n-butylacrylate. This material represents an interesting class of zwitterionic polymers, with large potential in reduction of biofouling of ultrafiltration membranes. It was found that the swelling rate and magnitude depend strongly on the content of zwitterionic groups incorporated in the polymer structure. Overshoot anomalous swelling dynamics were observed, explained by an interplay between enthalpic and entropic effects in various stages of the process. To elucidate the changes in the film properties a detailed ellipsometric analysis was conducted during polymer dilation and relaxation. For the polymer with 11.6 mol% zwitterionic groups, the thickness relaxation process after the swelling maximum is shown to be best described by a thin film with a time dependent uniaxial anisotropy. The behavior is rationalized via the orientations of polymer backbone and zwitterionic side chains, as a result of swelling and relaxation in the presence of the screening electrolyte. (C) 2013 Elsevier B.V. All rights reserved.