This paper airs to define the role of Christian Arab historians in influencing the substance of the information found in Muslim texts on Byzantium. Various Muslim historians such as al-Masʿudi, Ibn al-ʾAdim and Ibn Khaldun acknowledge their debt to the historical works of Arab Christian historians and an analysis of other Islamic sources reveals that they also must have been acquainted with, and actually dependent upon, Christian Arabic sources, especially in their treatment of the affairs of the Christian church, the theological movements and the ecumenical councils in early Byzantine history. For the later periods, the type of information on Byzantium that Muslim authors required and acquired is different. Much less is said about ecclesiastical affairs while a great deal of attention is focused upon various aspects of Arab-Byzantine relations. Thus, even though reliance on Christian Arab sources in malters pertaining to Byzantium continus, it is less systematic and more directly relevant to the Muslim context. This contrasts with the early period, when the Muslim view of Byzantium was fully shaped by the concerns of Christian Arab authors.