This article examines the thinking of Guy of Saint-Denis about plainchant tones as formulated in his Tractatus de Tonis (c.1300), preserved as the final item in an anthology of texts that he prepared (British Library, MS Harley 281). It examines his attitude to each of the major theorists singled out in this anthology. It argues that Guy's approach to chant combines the practically oriented writings of Guido of Arezzo with the Aristotelian perspective formulated by Johannes de Grocheio, but takes that perspective a step further by reflecting on the ways different types of chant impact on the emotions. Guy was also much influenced by Peter of Auvergne, a philosopher in the Arts Faculty at Paris committed to developing the teaching of Thomas Aquinas. Careful corrections to the Tractatus in Harley 281 reflect this ongoing concern to refine his thinking, possibly stimulated by Jerome of Moravia. His core conviction is that chant modes each have an affective attribute, and need to be chosen according to the subject matter of the text being sung. Guy criticised the practice of choosing modes sequentially in liturgical offices composed by those he calls ‘moderns’. Guy argues his case by drawing on examples of chant from Saint-Denis. A case can be made, on palaeographic grounds, for identifying him with Guy of Châtres, abbot of Saint-Denis (1326–42) and author of a Sanctilogium that updates the traditional monastic martyrology by reference to more recent Dominican collections of saints' lives in order to make them more accessible for liturgical use.