The effect of surface roughness on the quartz crystal microbalance with dissipation monitoring (QCM-D) response was investigated with emphasis on determining the amount of trapped water. Surfaces with different nanoroughnesses Were prepared on silica by self-assembly of cationic surfactants with different packing parameters. We used surfactants with quaternary ammonium bromide headgroups: the double-chained didodecyltrimethylammonium bromide (C-12)(2)-DAB (DDAB), the single-chained hexadecyltrimethylammonium bromide C(16)TAB (CTAB), and dodecyltrimethylammonium bromide C(12)TAB (DTAB). The amount of trapped water was obtained from the difference between the mass sensed by QCM-D and the adsorbed amount detected by optical reflectometry. The amount of water, which is sensed by QCM-D, was found to increase with the nanoroughness of the adsorbed layer. The water sensed by QCM-D cannot be assigned primarily to hydration water, because it differs substantially for adsorbed surfactant layers with similar headgroups but with different nanoscale topographies.