This article takes its lead from research into the ‘language’ of Roman portraiture. More specifically, it explores a work that literalizes the idea of ‘reading’ a Roman portrait (to quote Sheldon Nodelman's classic phrase): a picture-poem by Publilius Optatianus Porfyrius — a much maligned poet active in the first decades of the fourth century ad — that purports, through its iconotextual form, to visualize the countenance of the emperor Constantine (uultus Augusti). After a brief introduction to Optatian and his œuvre, the article offers a close reading of his third poem, demonstrating the sophisticated ways in which it probes the latent iconic potential of written script. What particularly interests me about this case study is its underlying paradox: on the one hand, Optatian boasts that his painted page will outstrip antiquity's most celebrated painter (it ‘will dare outdo the waxes of Apelles’, uincere Apelleas audebit pagina ceras); on the other, the actual form of the picture seems to eschew mimetic modes of representation, rendering Constantine's ‘portrait’ a geometric pattern. So how should we make sense of this image? What does the poem reveal about ideas of portraiture in the fourth century? And how might we contextualize Optatian's abiding fascination with the limits of ‘seeing’ and ‘reading’?