The paper discusses the questions of the alleged conversion of Arghun Aqa, the powerful Mongol governor of great parts of Western Asia in the mid-13th century, to Islam, claimed by the famous Armenian historian Kirakos. While in the end dismissing the historicity of this claim, the paper uses a variety of archaeological, numismatic, epigraphic and literary sources in order to highlight the great role the Islamic beliefs and identity of the surrounding Persianised society played in the continuous Islamic acculturation of Arghun Aqa and his family since the earliest phases of their presence in Iran.