The Annalista Saxo names "Welfum seniorum" son of "Azoni marchioni de Langobardia de castris Calun et Estin" (called "Welphus filius Azzonis marchionis Italorum" in an earlier passage and his wife "Cunizam"). After the death of his maternal uncle Welf III, he was summoned from Italy by his maternal grandmother to inherit the Welf family properties in Swabia and Bavaria.
He was installed in 1070 as WELF I Duke of Bavaria by Heinrich IV King of Germany. Duke Welf supported Rudolf von Rheinfelden, anti-king of Germany, and was deposed as Duke of Bavaria in 1077 by Heinrich IV. "Heinricus…rex" granted property of "Welfo dux dum erat dux…in pago Passir…in comitatu Gerungi et in comitatu Friderici" to the church of Brixen by charter dated 1078. "Dux Gewelfo eiusque…uxor Iudita" donated property to Kloster Weingarten, with the consent of "filiorum suorum Gwelfonis et Heinrici", dated 12 Mar 1094. He sought to reassert his position in northern Italy against Emperor Heinrich IV by arranging, through Pope Urban II, the marriage of his son to the powerful landowner Matilda Ctss of Tuscany. In 1095, Welf IV was reconciled with the emperor, who regranted him the duchy of Bavaria. He undertook an expedition to Italy after the death of his father in 1097 to assert his position in the inheritance over his half-brothers. Albert of Aix records that "Willelmus comes et princeps Pictaviensium, de sanguine et origine Henrici tertii imperatoris Romanorum" crossed Hungary peacefully with "duce Bawariorum Welfone et…comitssa…Ida de marchia Osterrich", entered the territory of the Bulgars in which "duce Bulgarorum Guz" refused their passage into Adrianople, but that Guillaume captured "ducem Bulgarorum" who was forced to allow the pilgrims to continue, undated but in a passage adjacent to text which records events in 1101. Albert of Aix records that, after their army was dispersed in Asia Minor by the Turks, Duke Welf eventually reached Jerusalem to complete his pilgrimage but died in Cyprus on the return journey.
Ekkehard records the death of Welf Duke of Bavaria and his burial in Cyprus. The necrology of Weingarten records the death "V Id Nov" of "Welf dux senior hic sepultus", which suggests that his body was moved after its first burial in Cyprus. 1
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