[1] In 1301, Wilhelm allied himself with the four Rhenish Electors who were demanding excessive customs duties for shipping on the Rhine river.
Having allied himself to the losing side of this war, Wilhelm lost part of his territory as a consequence.
[2] Prior to his death, Wilhelm established a Majorat, under which the eldest son was the sole successor and the county could not be further subdivided as had been done by his grandfather, Diether IV (died 1245), who divided Katzenelnbogen into Lower and Upper counties so that his two sons could each inherit.
After Wilhelm's death in 1331, the Lower county passed to his son Wilhelm II who died in 1385 with no issue and then to Wilhelm II's brother Eberhard V. When Eberhard V died in 1402, the Lower county passed to his only daughter Anna, who was married to Johann IV from the younger line, which resulted in the reunification of the two counties.
Wilhelm died on 18 November 1331 and was buried at Eberbach Abbey, which had been established as the burial place for the Counts of Katzenelnbogen by his uncle Eberhard I. Wilhelm's tomb was located in 1612/14 in the monastery church in front of the altar of St. John.