Fouling of membranes is a serious problem in membrane technology. By characterizing the foulants in membranes it is possible to understand fouling and reduce it. However, the characterization of foulants, especially organic ones, is difficult. The purpose of this study was to find out whether there are any organic foulants such as extractives in the membranes, and if it is possible to identify them. Membranes of different materials and hydrophilicity were used in filtration of ground wood mill (GWM) circulation water during one month in an integrated pulp and paper mill. Solid-liquid extraction was employed to remove the extractives from the membranes and the characterization of them was carried out with a gas chromatograph. According to the results, there are extractives in the membranes and it is possible to characterize them. It seems that the fouling by extractives mainly comes from resin and fatty acids. In addition, some traces of lignans were found in the membranes. Moreover, the hydrophobic membranes contained more of these acids and lignans than the hydrophilic membranes.