Arsenic is a ubiquitous element, which can enter terrestrial and aquatic environments through geologic and/or anthropogenic activities. Due to the toxic effects of arsenic to humans, it is necessary to understand the fate of arsenic species in environment. In this work, determination of four anionic species of arsenic in mobile fraction of river sediments from complex catchment was carried out in addition to total As content. Study was aimed primarily to assess the distribution of arsenic within the sediments of the study area. The relationships between mobile and total arsenic content and major physicochemical sediment properties were explored. The sampling strategy applied enables study of regional and seasonal changes in arsenic speciation related to mobility of As in aquatic system. The study area of typical complex catchment is located in the eastern part of the Czech. The whole watershed system is strongly affected by anthropogenic activities. The human influences in the area studied include agricultural activity, municipal waste and sewage plants, as well as the impact of the light and heavy industry.