Phosphorous (P) has been found to be the limiting nutrient in freshwater systems, directly affecting rates of planktonic growth. The P circulation is very complex, and its pathways through lake systems are difficult to determine. Motala Ström is the biggest watercourse in the south-east of Sweden and an important source of P to the Baltic Sea. The aim of this study is to conduct a P mass balance analysis of the lakes Roxen and Glan over a period of time. The analysis will also include a quality control of the concentrations data. The P concentration data used in this investigation were collected from the Motala Ström River Association, consisting of seasonal or monthly concentration data of Tot-P during the period 1960-2010. Daily water flow data used in this study were modelled by the Swedish Meteorological and Hydrological Institute (SMHI) using the S-HYPE model. P concentration deviations from monthly averages at each sampling station were calculated, followed by a seasonal Mann Kendall trend analysis. At five out of eight sampling stations, negative trends were detected, indicating decreasing concentrations. The exception was the outflow from lake Glan, Stångån and Finspångsån. Linear interpolation of P concentration data was performed to create daily data for the period 1980-2010. Following interpolation, daily transport values were calculated and summed up to annual values. Lake Roxen has acted as a source of P during the whole period 1980-2010, except for one year. Lake Glan has acted as a source during 22 of the 31 years. There is a tendency of Glan to become more of a source over the years, which is in line with the deviation observations, but variation between years makes it necessary to analyse also future data in order to establish any possible trend in P transports. Before construction of wastewater treatment plants, the lakes were certainly sinks of phosphorus. But at least for Roxen, the switch from sink to source was completed before 1980.