As a part of his policy of improving relations with the church in Rome, he persuaded Empress Irene to convoke, together with Pope Adrian I, what came to be known as the Second Council of Nicaea in 787, over which he presided.
Later in his patriarchy, he suffered from criticism of his alleged tolerance of simony, and of his handling of the divorce of Irene's son Constantine VI. Under severe pressure from Theodore of Studios, he excommunicated the priest who had conducted Constantine's second marriage.
When Nicephorus I dethroned Irene as Byzantine emperor, Tarasius crowned Nicephorus as the next emperor.
The legates of the pope and the oriental patriarchs being arrived, as also the bishops under their jurisdiction, the council was opened on the 1st of August in the Church of the Apostles, at Constantinople, in 786.
The patriarch, with good reason, judging the whole to be only an artful contrivance to impose upon him, answered that he was too well convinced that his passion for Theodota was at the bottom of all his complaints against the empress.
Tarasius persisting in his refusal to marry him to Theodota, the ceremony was performed by Joseph, treasurer of the church of Constantinople.