A monk, he was elevated to the patriarchal throne through the influence of the mother of the emperor Alexios I Komnenos, Anna Dalassene, to whom he had become an intimate advisor.
[2] Due to his illiteracy and apparent gullibility he was involved in the case of John Italus, whom his predecessor, Patriarch Cosmas I of Constantinople had condemned.
Alexios I had to take over the case against Italus as Eustratius, in his words, "rather dwelt at leisure and preferred peace and quiest to noisy throngs and turned to God alone".
This lack of resistance was not forgiven by Leo of Chalcedon who sought to expel him from his throne, at one point also accusing him, without evidence, of diverting part of the appropriate treasure for his own use.
[4] Finally, accused of heresy, Eustratius was cleared by a commission of inquiry established by Alexios I in 1084, but he chose to abdicate.