The paper argues that the debate with John Grammaticus in the fifth chapter of the Vita Constantini was a historical event. It is placed in the narrative immediately before Constantine’s participation in the mission to the Saracens, which is identified with that which took place in 855/856: this requires the emendation of a single character in the Vita, giving Constantine’s age as 29, not 24. (This also means that he could have been appointed chartophylax, and deacon, at 25.) It is suggested that the debate took place at the very end of 855, when it would have been relevant to political circumstances.