In the Leabhar breac bilingual homily (probably of eleventh-century origin) entitled 'In cena Domini' we find the curious expression Conuersio corporis et sanguinis [Christi] in panem et uinum, the exact opposite of what one would have expected. Since the Irish translation of this is quite different, and for us traditional ('the pure mysteries of his own Body and Blood under the species of bread and wine'), a simple scribal error might be suspected. This, however, is rendered less likely by the presence of the same unexpected formulation in Irish in another more or less contemporary composition in the Leabhar breac, the 'Instruction on the Sacraments'. The Irish Latin formula is probably best explained as a later development of such earlier Latin formulations as Transfiguratio [or transformatio] corporis Christi in panem et sanguinis in uinum, used in liturgical (with transformatio) and non-liturgical (with transfiguratio) texts. Early Hiberno-Latin exegetical and homiletic texts, in particular, make frequent use of the transfiguratio formula, and in contexts closely related with the Leabhar breac homily 'In cena Domini'. The present paper studies the general use of the liturgical formula and the Hiberno-Latin texts, and goes on to suggest ways in which this could have developed to give us the Leabhar breac Latin and Irish formulation.