In this study, homogenous dense chitosan membranes were prepared by solution-casting procedure. Then the membranes were ionically crosslinked by sulfuric acid. The surfaces of chitosan membranes before and after crosslinking were characterized by using FTIR-ATR, X-ray photoelectron spectroscopy (XPS), and atomic-force microscopy (AFM) techniques. The XPS data suggest that the surface composition of crosslinked membrane does not change significantly with respect to uncrosslinked membrane and the most important evidence is a certain amount of sulfur, coming from the crosslinker. The result from FTIR-ATR data shows the effectiveness of the cross-linking procedure by the shift in amide I and amide II bands. The investigation of membrane surfaces by AFM indicates that the crosslinking procedure modifies the surface morphology of chitosan. After crosslinking, the surface topography becomes more homogenous and relatively flat. © 2007 Wiley Periodicals, Inc.