Neutron reflectometry (NR) measurements have been made on thin (70-150 angstrom) poly(methylmethacrylate) (PMMA) films on Si/SiOx substrates in aqueous conditions, and compared with parameters measured using ellipsometry and X-Ray reflectometry (XRR) on dry films. All techniques show that the thin films prepared using spin-coating techniques were uniform and had low roughness at both the silicon and subphase interfaces, and similar surface energetics to thicker PMMA films. In aqueous solution, NR measurements at 25 degrees C showed that PMMA forms a partially hydrated layer at the SiOx interface 10 angstrom under the film, while the bulk film remains intact and contains around 4% water. Both the PMMA film layer and the sublayer showed minimal swelling over a period of 24 h. At 50 degrees C, PMMA films in aqueous solution roughen and swell, without loss of PMMA material at the surface. After cooling back to 25 degrees C, swelling and roughening increases further, with loss of material from the PMMA layer. (C) 2015 Elsevier B.V. All rights reserved.