Luxembourg became a grand duchy in 1815 when the Netherlands became an independent kingdom , and King William I of the Netherlands was made Grand Duke of Luxembourg . It has been a grand duchy since 1815 when the Netherlands became an independent kingdom and Luxembourg was handed over to the King of the Netherlands , William I. Luxembourg remained in personal union with the Netherlands crown until 1890 when William III , King of the Netherlands and Grand Duke of Luxembourg , died without leaving a male heir , so that in the Netherlands he was succeeded by his daughter Wilhelmina and , in the Luxembourgish Grand Duchy , by a distant male cousin , Duke Adolf of Nassau who became HRH Grand Duke Adolphe -- an arrangement necessitated by Salic law being applicable to Luxembourg but not to the Netherlands .