Basilios 10 was bishop of Magnesia ad Sipylum (in Asia); in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and was probably present at all eight sessions, from 24 September to 23 November 787: Mansi XII 995-XIII 486. In the list of those present at the first session he is styled Βασιλείου ἐπισκόπου Μαγνησίας τῆς Ἀνηλίου: Mansi XII 995. In the list of those present at the seventh session he is styled Βασιλείου ἐπισκόπου Μαγνησίας Ἠπιλου (sic): Mansi XIII 368. In the list of subscriptions at the fourth session he is styled Βασίλειος ἐπίσκοπος Μαγνησίας: Mansi XIII 141. In the list of speakers at the second session his name is omitted from the Greek list but in the Latin version he is styled "Basilius sanctissimus episcopus Magnesiae": Mansi XII 1097. The see of Magnesia ad Maeandrum was also under a bishop called Basilios (Basilios 14) but the two are usually distinguishable because the texts give the name of Basilios 14's see in full. However, in the list of subscriptions to the statement of the faith adopted by the Council at the seventh session there occurs only the entry Βασίλειος ἀνάξιος ἐπίσκοπος Μαγνησίας: Mansi XIII 385. The text is presumably defective, since both bishops attended the session and both doubtless subscribed the statement; an entry should therefore be added for Basilios 14. At the second session Basilios 10 spoke in support of the traditional veneration of icons as described in statements from pope Hadrian (Hadrianos 1) and condemned iconoclasts: Mansi XII 1097. At the fourth session he subscribed the statement read out from the Fathers in support of the veneration of icons: Mansi XIII 141.

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