The main objective of the project is a thorough analysis and comparison of all available GRACE time-variable solutions in order to study the changes in polar motion. The aim of such an analysis is to assess the errors in polar motion variations determined from different GRACE solutions as well as to study the agreement between particular series. The important part of the project will be an indication of the causes of discrepancies between solutions. This task will require a thorough analysis of computation algorithms and models used to develop GRACE solutions. The analyses will also include series from new GRACE Follow-On mission, not analysed before. Another objective of the project is to develop a combined GRACE solution based on currently available GRACE data in order to find a solution with lowest errors in hydrological excitation function. Our combined series will be unique since they will be developed particularly for the interpretation of polar motion variations.