During the minority of Baldwin IV, Count of Flanders, Eustace's grandfather, Arnulf III, Count of Boulogne, broke free of control by the count of Flanders. Both Eustace's father and Eustace himself remained independent. In 995, having reached his , Baldwin IV of Flanders tried to take back several of the independently held castles. He also expanded the Flemish borders. This had caused considerable between Baldwin IV, Count of Flanders and Eustace's father. When Baldwin IV's son Baldwin V succeeded him in 1035 Eustace I and Baldwin V of Flanders cooperated on several ventures including several charters. Eustace also signed charters of Baldwin V which limited the powers of the .