Upon the premature death of Henry in 1261, Adelaide assumed the regency on behalf of her underage son Henry. This arrangement was not accepted by all of the nobles at first as Adelaide faced opposition from Hendrik van Leuven of Gaasbeek, whom was a cousin of her husband. Despite this, Adelaide maintained her position until her son Johncame of age in 1268. One notable event of the regency was the change of Duke in 1267. Adelaide's eldest son, Henry, had physical and mental disabilities which rendered him incapable of effectively ruling the Duchy upon his majority. As a result, it was decided that the adolescent was to be removed from the position and be replaced by Adelaide's second son, John. Henry would go onto become a novice in Saint Bénigne Abbey in Dijon. It has been argued by historians such as H. Pirenne and B. Blumenkranz that Thomas Aquinas addressed his Epistola ad ducissam Brabantiae to Adelaide after she wrote to him, asking for counsel in regards to her Jewish subjects. In his final will and testament, her husband Henry had ordered for the expulsion of all usurers such as Jews and Cahorsins. In addition, Henry claimed that his Christian subjects should be freed from exactions. Although the then unknown Aquinas makes no explicit mention of Henry or his will and testament, his Epistola addresses how Adelaide should go about the matters raised by Henry. The work was only a minor treatise of Aquinas's however, it has proven significant with historians of antisemitism and taxation during the Medieval era, the two areas of concern. He suggested that Adelaide should tax Jews to a certain extent and make them wear recognition signs. Historians such as Raymond van Uytven doubt that the similarities between the two documents are a coincide, implying that Adelaide related her late husband's concerns to the theologian. Despite asking for Aquinas's advice, Adelaide never attempted to pursue her husband's demands. The treatise also offers the reader an insight into Adelaide herself. As the document is undated, some had suggested that the addressed Duchess was actually Adelaide's future daughter-in-law, Margaret of France. Although, no reference is made to a husband, suggesting that the Duchess in question wielded power independently of a spouse. Aquinas also writes of the Duchess's devotion to the Dominican friars. Adelaide backed this up by her founding the Château of Val-Duchesse priory for women in 1262. The name means Valley of the Duchess. It was the first priory for women in the Low Countries that followed the rule of Saint Dominic. In 1268, Adelaide handed over the reins of government to John and died five years later at roughly aged forty. One year later, Marie would become Queen of France.
